iron431/irons-spells-n-spellbooks#119
Reposted here because Irons-Spells-n-Spellbooks says it's Geckolib's issue

It's not their issue, yeah

But I'm not sure it's geckolib's either
I'd be looking at legendary tooltips for this

Either way I don't see what geckolib is doing that would be causing this at a glance - and since it's in legendary tooltips that it's broken, I'd be reporting it to them too