https://drive.google.com/file/d/1O6Q93nIXJmnlQMnAXmQtiqFWEfe9aNsi/view?usp=sharing
Also, There are linked files, so please use the BGtest.blend file
I don’t think anything you said would apply in regards to the differences of a render (frame) to a render (animation)
but feel free to try them both. with this cut down file of just the background (which I just tested and can reproduce the issue on two different computers with different setups and blender versions).