After updating the Android Gradle plugin from version 3.6.2 to 4.0.0-beta04 I get a ClassCastException in a release build:

2020-04-21 12:38:33.807 5147-5885/? E/AndroidRuntime: FATAL EXCEPTION: OkHttp Dispatcher
    Process: com.superthomaslab.hueessentials, PID: 5147
    java.lang.ClassCastException: i2 cannot be cast to ii8$a
        at li8.d(SourceFile:1)
        at vz4.a(SourceFile:4)
        at ak9.b(SourceFile:11)
        at bj9.a(SourceFile:23)
        at ak9.b(SourceFile:11)
        at li9.a(SourceFile:183)
        at ak9.b(SourceFile:11)
        at tj9.a(SourceFile:24)
        at ak9.b(SourceFile:11)
        at ck9.a(SourceFile:36)
        at ak9.b(SourceFile:11)
        at ak9.a(SourceFile:1)
        at xh9.c(SourceFile:24)
        at xh9$a.run(SourceFile:12)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1167)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:641)
        at java.lang.Thread.run(Thread.java:919)

The above stack trace retraced manually:

2020-04-21 12:38:33.807 5147-5885/? E/AndroidRuntime: FATAL EXCEPTION: OkHttp Dispatcher
    Process: com.superthomaslab.hueessentials, PID: 5147
    java.lang.ClassCastException: i2 cannot be cast to ii8$a
        at io.ktor.util.StringValuesSingleImpl.forEach(SourceFile:1)
        at com.superthomaslab.hueessentials.common.data.networking.KtorKt$addHeadersAfterRequest$1$$special$$inlined$-addNetworkInterceptor$1.intercept(SourceFile:4)
        at okhttp3.internal.http.RealInterceptorChain.proceed(SourceFile:11)
        at okhttp3.internal.connection.ConnectInterceptor.intercept(SourceFile:23)
        at okhttp3.internal.http.RealInterceptorChain.proceed(SourceFile:11)
        at okhttp3.internal.cache.CacheInterceptor.okhttp3.internal.http.RealInterceptorChain.proceed(SourceFile:183)
                                                   intercept
        at okhttp3.internal.http.RealInterceptorChain.proceed(SourceFile:11)
        at okhttp3.internal.http.BridgeInterceptor.okhttp3.internal.http.RealInterceptorChain.proceed(SourceFile:24)
                                                   intercept
        at okhttp3.internal.http.RealInterceptorChain.proceed(SourceFile:11)
        at okhttp3.internal.http.RetryAndFollowUpInterceptor.intercept(SourceFile:36)
        at okhttp3.internal.http.RealInterceptorChain.proceed(SourceFile:11)
        at okhttp3.internal.http.RealInterceptorChain.proceed(SourceFile:1)
        at okhttp3.RealCall.getResponseWithInterceptorChain(SourceFile:24)
        at okhttp3.RealCall$AsyncCall.run(SourceFile:12)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1167)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:641)
        at java.lang.Thread.run(Thread.java:919)

And another reported stack trace through Firebase (retraced automatically) which looks a bit different:

Fatal Exception: java.lang.ClassCastException: i2 cannot be cast to ii8$a
       at io.ktor.util.StringValuesSingleImpl.forEach(StringValuesSingleImpl.java:106)
       at com.superthomaslab.hueessentials.common.data.networking.KtorKt$addHeadersAfterRequest$1$$special$$inlined$-addNetworkInterceptor$1.intercept(KtorKt.java:87)
       at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.java:112)
       at okhttp3.internal.connection.ConnectInterceptor.intercept(ConnectInterceptor.java:37)
       at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.java:112)
       at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.java:87)
       at okhttp3.internal.cache.CacheInterceptor.intercept(CacheInterceptor.java:82)
       at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.java:112)
       at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.java:87)
       at okhttp3.internal.http.BridgeInterceptor.intercept(BridgeInterceptor.java:84)
       at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.java:112)
       at okhttp3.internal.http.RetryAndFollowUpInterceptor.intercept(RetryAndFollowUpInterceptor.java:71)
       at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.java:112)
       at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.java:87)
       at okhttp3.RealCall.getResponseWithInterceptorChain(RealCall.java:194)
       at okhttp3.RealCall$AsyncCall.run(RealCall.java:138)
       at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1167)
       at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:641)
       at java.lang.Thread.run(Thread.java:919)

This is part of the code in the stack trace (Ktor.kt):

package com.superthomaslab.hueessentials.common.data.networking

import io.ktor.client.engine.okhttp.OkHttpConfig
import io.ktor.http.Headers
// ...

actual typealias MyKtorEngineConfig = OkHttpConfig

actual fun MyKtorEngineConfig.addHeadersAfterRequest(headers: Headers) = config {
    addNetworkInterceptor { chain ->
        chain.proceed(chain.request())
            .newBuilder()
            .apply {
                headers.forEach { key, values ->
                    addHeader(key, values.single())
                }
            }
            .build()
    }
}

// ...

Some more details:

R8 version: 2.0.59
Android Gradle Plugin version: 4.0.0-beta04
android.enableR8.fullMode=true (does not crash when set to false!)
Device: Samsung Galaxy S10+ with Android version 10

See the attached APK which crashes. I can share the mapping / app bundle privately if needed.

Steps to reproduce in the attached APK:

Open the app
Tap Next until on the Connect Hue bridge screen
Tap Advanced at the bottom, tap Demo bridge, tap Connect
Tap Done at the bottom of the screen
Open a room by clicking the card view (for example Bathroom)
Go to the second tab (photo/gallery icon)
Tap the Fab + button at the bottom, tap Picture
The app crashes

I am trying to reproduce this in a sample project, but no luck so far.