OK, I think I figured out what was wrong in my setup. Appears that during the upgrade course I totally missed Garbage Collector settings in jvm11-server.options. After installation from RPM it defines the default CMS garbage collector, which is totally wrong for my hi-load environment. I returned back G1GC settings from my previous 3.11.x configuration and it seems it works as expected.
Thank you all, you're very helpful!
