Odd. Would have been interesting to see the network config prior to that reset.
Network didn’t really change between 2.21 and 3.0, assuming that’s the upgrade you did. If you upgraded from 2.0 to 3.0, then there may be more migration bits that could have gone wrong.
My usual suspect with those symptoms is dnsmasq failing to start for some reason, can be ip/port conflict or can be some files it couldn’t read. In either case, deleting and creating a new network would have reset that.