Could you tell, if the issue has appeared on the device, which was updated with Keep settings option enabled?
It was previously observed, that this issue happens on devices, which are upgraded from RUTOS 2.6 to RUTOS 7.0+ with Keep settings enabled, which would then result in mob1s1a1 and mob1s2a1 interfaces in mwan3 config have option track_ip configured, instead of list track_ip, which caused mobile interfaces to be incorrectly tracked. This should have been resolved with 7.3 version though.
Could you check and edit /etc/config/mwan3 file to contain proper options, if they are, in fact, incorrect?
Otherwise, could you reset the router to factory defaults?
If the issue persists afterwards, I would like you attach a troubleshoot file to your question. Make sure the issue is present before generating the file.