We seem to have run into this yesterday on a production server sing a custom compile of the 5.2.9 buster-backports kernel. nginx was hung in D status, sync hung as well, no obvious reason for it; I ended up having to reset the machine.
On boot I found we had lost several hours of logs and worse, several user data files supposedly saved during that time. A small but noticeable increase in iowait accompanied the start of the lost logs and continued until the hang.
I guess I was lucky we did not lose more - at least, I *hope* not, because it is hard to be sure of that without a byte-by-byte verification...
Don't know if it is related, but the behaviour of this kernel has also been to significantly increase the size of the slab cache of dentries/inodes during rsync, resulting in fragmentation and a massive increase in the amount of free memory (despite setting vm.vfs_cache_pressure=10000 in an attempt to remedy it).
I can of course compile my own kernel, but I concur with
Christoph's assessment of this bug's severity and would encourage a new backports release ASAP.--
Laurence "GreenReaper" Parry - administrator, Inkbunny.net