this is to be expected, because sysupgrade restores your old switch config which becomes invalid with the new fw.
you can either use a serial console to fix it after an update.
or patch your target/linux/alpine/base-files/etc/board.d/02_network and then use factory fw flash.
i tested the new switch setup with both my r9000 and xr700 routers, and everything works.
i tested everything myself before i posted the instructions here.