Hey,

I tried to convert an APNG with Pillow and got a bad looking output. It seems that WEBP saving works a bit better but there are also problems. This issue is about converting an APNG into an APNG while having the same watermarking/thumbnailing as in #4644. I've attached a self-contained test code apng-test.zip to reproduce the problem.

The used APNG is downloaded from https://en.wikipedia.org/wiki/APNG, the wikipedia article about APNG formats.

Hope to get this fixed soon, because I have to convert all APNG-s into WEBPs to be able to look half decent.