Zyxel Armor Z2 Snapshot build partition table problems

Hello Members!

I have a strange problem with my Zyxel Armor Z2 for the last two updates, which I could only resolve by downgrading to the stable (now 19.07.7) revision, but that excludes WEP3, which I used for some time now.
The issue started with an upgrade to the r16237 build (from a snapshot version I cannot recall which was but not older that 1 month), and when ssh-ing to the router to do the updates and install the things I use it, it greeted me with the basic login as it should when I install it fresh and had my router completly reseted.
Of yourse I do have backup of my config, but this was unexpected. An every time I did a backup restore, after rebooting, the router was basically sweeped clean and all was reseted every time.
After some initial trying I did a standard df to see what the hell is going on, and seen this:

Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/root                 4096      4096         0 100% /rom
tmpfs                   238952        68    238884   0% /tmp
tmpfs                   238952        92    238860   0% /tmp/root
overlayfs:/tmp/root     238952        92    238860   0% /
tmpfs                      512         0       512   0% /dev

Seems like overlayfs is somehow pointed to /tmp so of course it gets deleted every time i reboot the thing, and afer going back to the up-to-date stable release (19.07.7), df shows the following:

Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/root                 3840      3840         0 100% /rom
tmpfs                   240228     22424    217804   9% /tmp
/dev/loop0               55765     13204     38234  26% /overlay
overlayfs:/overlay       55765     13204     38234  26% /
tmpfs                      512         0       512   0% /dev

In here I can clearly see that the fs table is different, and also sizes are better and adds up.

I do not know what caused this, and I do not know why, but it is rendering the snapshot buillds unusable for me at least. I do not use double-boot and has not tinkered with the eMMC so the device is as stock as it is except for openwrt.

I would like to ask to whom can help me to how this issue can be resolved? I know basic linux, but not to this extent.