Players report a bug on live servers that causes raid and Mythic+ groups to disband randomly in the middle of runs.
According to reports, the most-affected server experiencing issues is Tarren Mill-EU, where a raid group disbanded mid-fight during Mythic Terros progress, and more.
Another Mythic+ group on Twisting Nether got disbanded and the list goes on.
Those were all reports from 3 days ago. However, the issue is still ongoing with reports from both EU and US players.
Hopefully, Blizzard will be able to backtrack and fix the bug soon.