Removal of thread__ for fc_cache and fi_cache do not help. The issue is still there.
As for the testcode, I could not spot any false characters there, but the granularity due to significant zooming ruins part of the larger texts.
It looks like this change of removing thread__ for fc_cache and fi_cache did not get compiled for me as I was simultaneously working on the testcode. Now I recompiled all and tried this once more and BANG! The problem was gone from my own program. And I also got it back by adding the thread__ ahead of those two caches. I think this solved the issue!
I must fly now... Will be back for more testing tomorrow.
A million thanks and best regards,
Obviously the next step is to try whether it is caused by first, second or both...