I don't follow this thread that close as I don't have 860l anymore, however there is this in the 19.07.4 changelog:
Device support: images for some device became too big to support a persistent overlay, causing such devices 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