I finally have some (good) news.
They managed to reproduce the issue, but only on Mox devices that have an old U-Boot version.
So they asked me which U-Boot version I have on my board. Mine is 2018.11. You can check yours with the following command:
root@turris:~# strings /dev/mtdblock1 | grep "U-Boot 20"
U-Boot 2018.11 (Dec 16 2018 - 12:50:19 +0000)
root@turris:~#
Their conclusion:
Dear Mossroy,
this means that you have old firmware, which is the cause of the issue.
Please install the nor-update package and run it, with commands:
opkg update
opkg install nor-update
nor-updateWe will be sending you back your USB flash drive on monday.
I haven’t tested yet.
Do you have the same U-Boot version as I have?
@jada4p can you check you already have a more recent one? (which would explain why the issue was solved for you since TOS 6.2.2)