Curse comments:
https://www.curseforge.com/wow/addons/worldbosstimers?comment=151
https://imgur.com/a/KXnsWC0

It looks like:
He just killed Rukhmar on Rag:Nor, but there was an old timer for Rag:Nor that did not disappear.

I myself have never seen this happen though. Something might give the kills different IDs that I have not considered.

Old timers now get pruned at addon load time if they do not match latest GUID pattern. There have been fixes related to this as well, so hopefully fixed by now.