I noticed this issue started with 12.4. Arathi Footman are casting Defend and my custom plate shows correctly. I knock back to interrupt and the defend bubbles are gone, but the tptp castbar continues. I checked using default warcraft castbars and they do disappear after I interrupt.

See images for specifics.

Settings

Casting

After interrupt (bars still showing)

That looks like a bug, definitely. In the second image it also looks like not only that the castbar is not hidden (it gets stuck), but also that the custom nameplate does not reset correctly (to non-custom). Am I mistaken or is it like that?

I can verify this defect.

I uploaded a new beta version that should fix this issue.

What is weird is that I could also create this error in Priory of the Sacred Flame with Arathi Footman, but not outside of an instance, e.g., with other mobs channeling non-interruptibe casts.

Internally, the WoW API still returns channeling information after the channel was interrupted, which should be wrong, but I did not find any bug report regarding this. Still, I fixed the bug by optimizing some code that was not ideal or correct anyway :D

I found this only in Priory as well?? Strange behavior. I was able to test the beta version with Priory/Defend and Ara-Kara/Alerting Shrill and both worked great. Thanks for fixing this so quickly.

I am not fully sure, but this might be related to this WoW issue (on retail): Stanzilla/WoWUIBugs#777. So it could be a WoW issue at its core.