I can confirm safe mode still autoscrolls. I know you were unable to replicate this on a test env, but does the test env include the bottom bar. I suspect if the bottom bar is in view of the screen then the thread will auto scroll after some testing.
May be a red herring but if this is not in view on the page then it doesnt seem to occur. If I scroll so that this is totally out of view but I can still see the latest post it does not occur. If even partially visible it will autoscroll.