The good news is that I've investigated it and I know why it fails and I also know what shall be done to make it work.
The bad news is that adding support for such APNGs, with good performance, is considerably more complicated than the current implementation.
There are two types of APNGs, there are images where each frame replaces the previous one and there are images where the images are stacked in all kinds of ways. For now, only the first type is supported.
I will add a message about it.