With Flush connections set to connected and disconnected, on mobile disconnection openVPN should be rerouted through the other WAN interface.
In your failover configuration in Network -> Failover page, which interface has a lower metric value (lower metric means higher priority)?
Also, could you update your device to RUT2_R_00.07.04.2 with Keep settings option set to off, reconfigure it and check, if the issue still persists.
If it does, I would like you to attach a troubleshoot file to your question. Please, wait till mobile and OpenVPN disconnect a couple of times to have log entries in the file, then access router's WebUI, go to System -> Administration -> Troubleshoot section and download troubleshoot file from there.
Best regards,