Sorry for long response. I was on a trip during last 5 days.
What I have found:
Lets take a look on this patch [1]. It must be a contribution of gengchc2, but for some reasons it was matched to Yuval Brik [2]
I'm still trying to find a root cause of it, but anyway on this week we are planing to rebuild our database to increase RAM. I checked statistics of gengchc2 on clean database and it's complete correct.
So your problem will be solved in several days. It will take so long time because full rebuild of DB takes 48 hours, but we need to test our migration process first to keep zero down time.
I'll share a results with you here when the process will be finished. Thank you for your patience.
Sergey