After the move to the new "fip in ubi" layout, I ran into bogus compatibility problem in the next sysupgrade:
I noticed at the next sysupgrade a few days later that it complained about the sysupgrade image compatibility. I had to manually add the compat_version '2.0' option to /etc/config/system.
The reason for the complaint is that although the first sysupgrade image uci-defaults had already set the compat_string, I had restored settings from backup after the fip/ubi change, and the restored old settings backup had naturally not contained the new string, and so the string "disappeared" from uci.
The string in the system section of the system config file can be added by editor, or by UCI commands.
uci set system.@system[0].compat_version="2.0"
uci commit system