Scanclass OPC Data Mode - Subscribed causes bad quality

Our scanclasses are driven by tags getting values through UDP (OPC-UA/Devices/UDP Driver). Different UDP port for every station telling scanclass when to go to the fast rate. It happens by event from controller, otherwise it waits for 400 seconds. After 400 seconds all scanclasses go to the fast rate. I have attached screenshot. Scanclasses provider is [default], so it’s internal.
OPC Data Mode is Read for Modbus TCP devices. If we assign Subscribed for Modbus TCP devices, after tag read session (when scanclass goes to fast rate) tags qualities go bad for couple seconds for some reason.
Modbus TCP devices are defined under OPC-UA/Devices. Devices are using 3G connection.

Could you please tell, why Modbus TCP tags qualities go bad for couple seconds when scanclass goes to fast rate?