The issue was that during playback, the evaluated Grease Pencil ID
is not being copied again for re-evaluation. This meant that the
adjustments get re-applied over and over again.
Ideally, we would restore the drawing pointers before modifiying them
again, but this is a bit trickier to implement.
To fix this issue for now, we do the layer adjustments during object
data evaluation. Using the geometry set for modifier evaluation
means that we copy the data and apply the adjustments for every
object.
In the future this can be optimized to only happen once per
Grease Pencil ID (in `BKE_grease_pencil_eval_geometry`).
Pull Request: https://projects.blender.org/blender/blender/pulls/135387
c24b540400