https://yurie.thigh-high.club/u5q97zs4.mp4

Even default animated BTTV emote does that.

I assume it's because of how ImageLoader handles the GIF emotes, especially applying one disposal method per image instead of doing that per frame.
(to be honest, i'm not even sure if doNotDispose even works in the current implementation)

Here is my test information about the three gif emotes on the video above.

Closing since the mod is no longer supported and will be rewritten once ported to modern Minecraft version.