Create a new world, add an ender eye to inventory, triggers UB advancement toast notification, but does not unlock any UB entries (as I would expect it to). The toast notification is firing when it shouldn't.

The toast notification is brief. I'll keep an eye on it and report back.

To be sure is the toast the "new entries unlocked" or an "advancement unlocked" toast? If it's the former I remember having some similar issue on other patchoulis so the issue might come from there. Either way I'll try to check when I get some time.

EDIT : I quickly checked if I could get the issue to happen, and I couldn't get it to happen on both the dev environment and throwing latest UB + Patchouli on my old 1.12.2 test instance, so it's likely another mod is causing the issue in some way.

Could you see if it still happens with just UB + Patchouli, and/or give the mod list where it does happen?

It's the "new entries unlocked" toast.