Upon configuring openwrt for build I noticed 'rootfs snapshot' tool in configuration.
Curious I checked what it is, and by looking at https://github.com/openwrt/openwrt/blob/master/package/system/fstools/files/snapshot it looks like it's not working/needed/maintained part of fstools.
For example it's using /volatile/upper as a checkpoint, and SNAPSHOT=magic variable -> which are not proper/not working anymore.
as it's in the 'base' I think it would be worthwile to fix/clean it. If somebody points me in the right direction I'm willing to help with it.