If you have failover setup, if the default Sim’s signal is too weak, it would change to another Sim, that is the default behaviour. Are you saying it has switched to another Sim, but the change doesn’t get locked in place? Have you tried using the device only with either of the sim’s? Are you able to get internet connection with both of them?
The “SIM state changed to not inserted” is an internal modem state during SIM slot switching.