On openSUSE Leap 15.1, default Plasma 5.12.8LTS with Frameworks 5.55.0 and Qt 5.9.7.

From within the Plasma Bluetooth module I linked to my phone and set up a dun connection, which I subsequently got working. It then appears in the plasma-nm connections editor, but when changing any option for that connection, the Apply button remains greyed out. It is not possible to effect any change with the OK button or by selecting another connection and receiving a prompt. If I OK the dialog, when I return none of my changes have been saved.

The most troublesome effect of this is that the connection also automatically assumes priority and keeps trying to connect every few minutes, regardless of whether I already have another active and stable connection, and regardless of whether bluetooth is activated on the device. This could result in data being unknowingly transferred over my phone's bluetooth connection, with myriad consequences, not least an unexpected huge phone bill! Since no firewall zone has been selected or is indeed selectable when creating the connection, this also opens up potential security issues.

I've been able to resolve the problem after editing the file relating to the connection in /etc/NetworkManager/system-connections/ by adding these lines under the [connection] section:
autoconnect=false
autoconnect-priority=-1
zone=external

and then rebooting (probably a restart command would have done the trick). But other users could be unaware that this dun connection is poaching priority and transferring their data, or indeed they may edit the connection, click OK and think all is well.