The "EXT4 Corruption Issue" Has Been Fixed In Linux 4.20, Backport Pending To 4.19
As outlined yesterday, this "EXT4 file-system corruption problem" was actually an issue in the BLK-MQ code within the kernel's block subsystem. That article yesterday has more background information and that problem in the multi-queue block I/O code indeed turned out to be what was causing the problem that's been happening going back to Linux 4.19-rc1 and persisting through the various Linux 4.19 point releases and also Linux 4.20 Git. Disks on Linux 4.19+ were only vulnerable if using BLK-MQ and using no I/O scheduler.
Fortunately, as of this evening, Jens Axboe submitted a pull request with the fix and it's now landed in Linux 4.20 Git. Axboe commented, "Under a combination of circumstance, the direct issue path in blk-mq could corrupt data. This wasn't easy to hit, but the ones that are affected by it, seem to hit it pretty easily. None of the regular filesystem and storage testing has triggered it, even though it's been around since 4.19-rc1."
The patch is quite small and should be back-ported to Linux 4.19 for its next stable point release, which will be Linux 4.19.8. The code did not land in time for today's Linux 4.19.7 point release.