This has been happening for awhile for me, but it's become very annoying in the latest versions as use more groups especially. I found a different problem with groups that I will post as a seperate thread.
I usually have about 6 browser windows open. 4 for personal profile, 2 for work. Each has multiple tabs open in it and at least one work tab and one personal tab have groups in them of multiple tabs each. One of my personal windows also keeps a pinned tab open for outlook.com and feedly.com.
Lately after a browser restart event when the browser asks if it should restore my prior session, it moves the pinned tabs and the grouped tabs in the personal profile to random restored window sessions. Sometimes it's the same as it was before the restart event, oftern time it's not.
This is super annoying.