Taunt as an interrupt
I was playing in WvW on my Ranger, using the Beastly Warden trait which causes pet F2 skills to taunt nearby enemies. Some NPCs (eg checkpoint sentries) use a self-heal with an obvious animation of sticking one hand up in the air while channeling it. If I hit them with the F2 taunt when I see that animation I get the “interrupt” message, but their heal still goes off.
I’m not sure what the intended functionality of Taunt is, whether the interrupt message shouldn’t be displayed, or whether their heal should be interrupted, but either way there’s something going wrong.
Does the interrupt work with other foe skills (most of which also have obvious animations)?
Some skills have multiple effects, some channeled and some not. It’s possible that the heal skill in question has that, in which case you might interrupt the the channeled bit but not the instant bit.
Or, as you surmised, it could be a bug. Best way to make sure ANet’s QA has the details they need to troubleshoot: use the in-game /bug tool to report it right after it happens (or as soon as is ‘safe’ for you to do so). That will allow them to see actual game data for the time of taunt.