jffs2 errors with 5.4 kernel

[Nick]

At the first start after flashing, there is often a "mount_root: failed
to sync jffs2 overlay2".
This problem only occurs with the 5.4 kernel. If I build a 4.19 kernel
with master, it works.
We have tested
- FRITZ!Box 4040 (ipq40xx)
- Nanobeam AC Gen2 (ath79)
- Nanostation AC loco (ath79)

On the UBNT devices it is even worse, because the filesystem is broken
after the first flash and does not keep changes after a reboot. There
are even other problems, such as the root password being incorrect.
A typical log after a reboot log contains several: "Newly-erased block
contained word 0x0 at offset 0x00420000"
More Infos and logs for ubnt devices:
https://bugs.openwrt.org/index.php?do=details&task_id=3272

Logs AVM Fritz!Box:
-
https://gist.github.com/PolynomialDivision/96bfd98d723f288c617fd96f8ff04c22

I think many more devices are affected. Keep an eye out for jffs2 errors.

We believe that the mx25l12835f (16-pin package) chip is causing the
problem.

OpenWrt Forum:
https://forum.openwrt.org/t/nanobeam-ac-gen-2/70640/26