There are variations of this bug. Some examples are posted below. Some of them (the videos) go back to Season 6. Some are from the PTR and some are from the Live game. This has been an issue for a long time. Fortunately, it is usually an occasional issue.
This Season, yesterday as a matter of fact, I encountered the monster bug shown in the third video (above) while running a Greater Rift. This time the entire GRift Level was populated with these Morlu. About one-third of these monsters were able to be killed. The other two-thirds could not be killed. Luckily, they do not pursue or attack once you kill them the first time.
I did not video capture yesterday’s issue. And it is the only time, so far, I’ve experienced it this Season.