Oh, the annoying part beyond having to find and re-group them is that all contextual comments, linked bugs (etc?) are lost.

I might have found the reason why because one of the bots failing was pushed down to the "new failures" section and that one still had the comments and linked bug. I didn't notice it at first as all others were kept within "consistent failures".

My theory is that a new failing layout test in this specific both might have caused the automatic un-grouping. It used to be that only 

plugins/webview-plugin-type-change.html

was consistently failing but the last (and only the last)_completed build also had

http/tests/devtools/console/console-viewport-stick-to-bottom.js

failing.

By "the last (and only the last)" I meant that it was the first time that new test failure happened.

And for some reason it won't let me group that one back with the others. Maybe SoM is doing that because it avoids grouping of failures with different test sets? It is currently assuming the only failure is that 2nd test but I already confirmed (from the next unfinished build) that it was just a fluke while the former is still consistent.

Thanks for filing this bug! I believe this is from annotations failing to load at times.