I'm having the same issue but couldn't find the exact spid that caused the tempdb growth. i killed the sleeping sessions.
Also, we used to get corrupted tempdb stats on read only databases in always on group. every time we get that issue, we have to drop the corrupted stats and query optimizer will re-create them again. We reached out to Microsoft support and they are working on a bug fix.
But for version store space growth in tempdb , i'm not sure if that was because of temporary stats ? Also, there is only one database that has RCSI enabled out of 15 other other databases that are part of always on. I have just enabled RCSI on 3TB database yesterday because of blocking issues. not sure how this is going to behave, although we have more than a TB space on tempdb but still after looking at the version store space yesterday (growth was 750GB), i'm kind of concerned.
Any help on this is greatly appreciated.