Using the "Bee Wither" resource pack from Vanilla Tweaks (https://vanillatweaks.net/picker/resource-packs/ the "fun" section) The wither boss bar was not hidden when the muffler was right clicked.
mc 1.18
forge 39.0.88
MGU 0.4.33

I'm assuming it similarly doesn't work for the Ender Dragon if the LANG file renames it, seeing as taking that line out of the English translation for this resource pack resolved the issue.

Problem is, because of the way boss bars work, all we have is the translated name to go by, only thing i could do is make a list of alternate names to check.

Would you be able to put in a wildcard to catch anything with wither in the name?

hmmm, ya that would probably work.