The Ubiquiti UniFi controller allows managing Ubiquiti devices on your network via a web browser. For this it queries the devices via snmp which requires the network-observe interface to be connected.
The snap ships its own mongodb binary which tries to determine the file system type it is running on during setup to pick the correct default profile for the configuration it generates. To access this information it needs the mount-observe interface connected before first startup.
Please grant mount-observe and network-observe auto-connections for this snap.