Frequently a monster becomes almost unkillable for about 5-6 seconds. His life points decrease very slowly (if at all) during these 5-6 seconds and after this time, he behaves normally. And it has nothing to do with the level of the monster (ordinary watch, elite, champion, rift guardian.) nor the level of the game (normal, torment 1 to 16) and occurs anywhere ( normal zones, Nephalem rifts, Superior rifts, etc.). I even thought it was coming from the ‘stun’ of my follower the Scoundrel, but the bug is always present, no matter which follower I use or none.
An online player told me that the bug has been known to Blizzard for years.
Can we hope for a correction of this bug? It wastes a lot of time to flee for 5-6s to wait for the monster to ‘debug’ and at high level it is often assured death from a monster that casts violent spells before you can move away to wait for the monster to ‘debug’.
Please give an answer.
Bonjour Wakanidoo, 
To report a bug in Diablo III, please use the Diablo III Bug Report forum:
You’re not going to get a reply from Blizzard but you may get one from the regulars on that forum.
Alternatively, you could post in the Wizard category and ask the players there if they ever experienced that type of an issue… and whether there is a workaround.
The forum here is for technical support on Blizzard’s Classic Games — old games such as Diablo II: Lord of Destruction. Diablo III does not (yet !) qualify as a legacy game.
Best of luck in your games !
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Thank you Boubou, i got the answer in the link you gave to me:
7. Unkillable monsters. This is also random and long standing. No work around.
Clearly, Blizzard dont care 