I have tracked the issue to this change:
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/diff/block/blk-flush.c?id=344e9ffcbd1898e1dc04085564a6e05c30ea8199
The commit log message makes it seem that it is a refactoring change,
but it inverts the sign on q->elevator test. It should have been
documented as a bug fix and backported to stable branches.
The line was not further modified in the "master" branch.
I have rebuilt 4.19 from Debian testing with this change, and my system
now works correctly.
In my opinion, it also relates to #913138 and maybe #904822.
Dragan
