Update: So disabling GTK rekeying in the SSID/network settings has changed the situation, but not fixed it.
Now the affected shelly's are only connecting to the old AP-AC lite unit, which is not good because they have less than 20% (-83dBM) signal. Back last week when they connected to the U6-Lite they had >90% -46dBm signal.
When I turn off the AC-lite to force the Shellys to connect to the U6, they appear to connect (according to the Unifi controller UI), but are not accessible at their ip address and don't respond to pings.
Shelly FW: 20210909-145300/v1.11.4-DNSfix-ge6b2f6d
U6-Lite FW: 5.60.9
UAP-Lite: 5.43.43