There is a theory that the flash bug might have been fixed when the flash driver was rewritten in 4.16. They have been trying to reproduce with 4.19 and not succeeding. However, it is possible that they aren’t driving the boards hard enough to reproduce.
My board only reboots when I’m testing a new kernel. It has been up for weeks at a time when I wasn’t trying kernel patches. I do lots of make -j4 builds and checks. I’m using NBD for root and running without swap at 1GHz. If you are using flash for swap, that might be causing the problem.