I have not seen it myself, but I thought I remembered another thread talking about this. Here is a link. The second link is to the Bug Report someone made for it.
https://us.forums.blizzard.com/en/wow/t/aggro-after-wipe-bug-with-frost-mages/507443
https://us.forums.blizzard.com/en/wow/t/aggro-after-trash-wipe-bug-w-frost-mage/723376