Apologies for the lack of crash report - on mobile and just remembered this issue. I’m sure the reason it crashes is self-explanatory though.

Their mixin is here:
https://github.com/apace100/apoli/blob/1.21.x/dev/src/main/java/io/github/apace100/apoli/mixin/LeashableMixin.java

It looks like you’re implementing similar logic, maybe you could use a similar Inject+WrapWithCondition pattern?

I've made my code similar to apoli's, looks like it works.
I haven't been able to test it myself with Origins/Apoli though.