https://madustrialtd.asuscomm.com/unrealforum/index.php?resources/unrealcore-mod.2/
https://madustrialtd.asuscomm.com/unrealforum/index.php?resources/unrealexample.3/

can you try to fix it?

Here are the files for testing the server with this mod and your
https://drive.google.com/file/d/1mL8svxHiRRRFBVQzOepnB1KF9G5d3baL/view?usp=sharing

Summon a mob using the /mm egg get command

mods.zip

Why are you not taking this to unreal core?

Why are you not taking this to unreal core?

I'm gonna close this and leave it to UnrealCore to debug

Given that it only happens to their mod (I've never heard of this outside of mods doing very poor implementation of models/animations), and I'm unable to replicate this at all on my end, and no-one else has ever reported this, I'm going to assume it's something to do with their implementation of the model or animation, or some related facet.

To summarise: This is not A GeckoLib issue as far as I can tell. This seems to be specific to their mod(s) and model.