On Pixel 3a, setting flash to automatic results in dark photographs, as the second flash to light the scene doesn't trigger.

Setting flash to manually on and taking a photo will have a long flash for the autofocus to work, then a second shorter flash as the photo is taken.

Setting flash to auto and being somewhere dark enough to require the flash will have a long flash for the autofocus, but not then trigger the second flash whilst the photo is taken, resulting in a dark picture.

To test/debug this issue you'll could use our CameraX based app (releases section) on a Pixel 3a: https://github.com/GrapheneOS/Camera

Thanks a lot for all the help and support.

Regards, Mohit