Winbox of MT v. 2.9.2 has a bug. It crashes when I’m trying to close PPPoE tonnel with router which I’m managing just now. I think it should show “connection lost” instead or just stay working.
It happens only with PPP or Interfaces window open, when it must disappear from list.
Please, upgrade your router to 2.9.11 version, and download the latest Winbox version from router.
It should resolve requested problem.