>> Since root cause of the problems is more or less known -

> Oh, that's good, I was not aware. 

Sorry - somehow the feedback got lost. Last communication from Reinhard Max was that Postgres has nasty bug related to glibc upgrade, which e.g. happens during Leap upgrade 15.2 -> 15.3
It affects both pg 12 and pg 13 and proper workaround is to rebuild all tables with unique string indexes.
Since all tables were freshly created at mirrordb2 when restoring logical backup - mirrordb2 should be good. But it doesn't require to stick to pg 12 - it should be safe to upgrade it to pg 13 now.