Found the root cause. For some reason, this firmware brought back an old incompatibility with routers supporting a combination of WPA3+WPA2 authentication which has been resolved long ago in most current Garmin devices.
Switching the access point back to legacy WPA2-only mode for 2.4G re-enabled the connection again.
n.b. that this does not solve the "Sync Failed" error message which is independent from Wi-Fi connectivity, though.
