I know the answer to this. The ext4 filesystem that openwrt creates by default is just janky. To get well-working filesystem that you can resize etc, you need to build custom image with a small patch to fix up the ext4 fs creation.

https://github.com/openwrt/openwrt/issues/7729#issuecomment-...

https://github.com/openwrt/openwrt/compare/main...zokier:ope...

This is exactly the sort of thing why I'd want a openwrt for "big" devices. But I should get that patch also merged upstream...