Sounds like this known issue, mentioned in the release notes:
Images for some device became too big to support a persistent overlay, causing such models to lose configuration after a reboot. If you experience this problem, please report the affected device in the forum and consider downgrading to OpenWrt 18.06 or using the Image Builder to pack a smaller custom image.
Images for some device became too big to support a persistent overlay, causing such models to lose configuration after a reboot. If you experience this problem, please report the affected device in the forum and consider downgrading to OpenWrt 18.06 or using the Image Builder to pack a smaller custom image.