IssueTracker

Component used: 'androidx.lifecycle:lifecycle-extensions:2.0.0'
Devices/Android versions reproduced on: robolectric, emulator, real devices

I have an activity, and a fragment in it (MasterFragment).
Than I add nested fragment (NestedFragmentWithViewModel) to a MasterFragment.
NestedFragmentWithViewModel obtains viewmodel by `ViewModelProviders.of(this)`
Than I replace it (with adding to a backstack) with another fragment (NestedFragment).
After that, activity gets recreated, which is causing:
 - MasterFragment to recrete
 - NestedFragment (as it is on top of backstack) to recreate
After that, activity gets destroyed, which is causing:
 - MasterFragment to destroy and clear its ViewModelStore
 - NestedFragment to destroy and clear its ViewModelStore

But no one cares neither about NestedFragmentWithViewModel in the backstack, nor about clearing its ViewModelStore.
It is not cleared when fragment is added to a backstack (cause backstack can be popped to show fragment again).  
And it is not cleared when MasterFragment destroys, cause it calls onDestroy on all its 'created' nested fragments, but fragments in a backstack were not created after activity get recreated, so should not be destroyed.

Fragment's ViewModelStore is cleared in fragment's onDestroy() in case activity.isFinishing(). That never happens so, whole ViewModelStore gets leaked.

It is not a totally FragmentManager issue, cause its respects lifecycle of fragments - 'what is dead may never die'.
So it seems fragment's logic to clear its ViewModelStore should be fixed.

Project with described scenario is attached.