I did rebuild debian kernel (linux-source-5.4 == 5.4.19-1~bpo10+1) with ubuntu 5.4.0-26-generic and it works again. And tested kernel for a few days - no corruption!
Looks like something wrong with original debian config which causes random data loss on my SSD. Attaching diffconfig's.
config-5.4.19 - debian source code compiled with ubuntu config
config-5.4.0-0.bpo.4-amd64 - debian default kernel
config-5.4.0-26-generic - ubuntu kernel
/usr/src/linux-headers-5.4.19/scripts/diffconfig config-5.4.19 config-5.4.0-26-generic > ~/debian-5.4.19-ubuntu-5.4.0-26-generic.txt
/usr/src/linux-headers-5.4.19/scripts/diffconfig config-5.4.0-0.bpo.4-amd64 config-5.4.19 > ~/debian-5.4.0-0.bpo.4-debian-5.4.19.txt
Maybe anyone can suggest which configs / values should I try to enable / disable.