I fixed it!
I deleted the code snippet around line 700 in fossil.js, they were about the animation.
What prompted my decision came from the comment: //we can't just grab the animation duration in RMMZ the way we do in RMMV
//because effekseer doesn't let us just grab animation.frames.length
//so we have this horrible hack where we check for the last sound or the end
//of the last flash instead.
//if you want to extend an animation,
//just set a flash of duration 1 frame at the end point
Although I know fossil.js allows adding code snippets, thus fixing the problem.
But I'm not familiar with it and I'd appreciate it if you would tell me how to do it.