Having WMC installed on the same PC, whether it is running or not, can really muck up MC TV functionality and tuner availability. That is because WMC will still check tuners occasionally, and will use them for EPG updates when it wants to, without checking if they are already in use. WMC and all its services need to be completely shut down for MC to work properly. Or just uninstalled, which I guess will happen as soon as you update to Windows 10.
Regardless, that shouldn't have changed the recording format to JTV. Could the JTV files have been Time Shift files, rather than a recording? Because MC always saves Time Shift files in JTV format, and then deletes them later, even when recording in TS format.
Mind you, if MC was able to save Time Shift files, that meant it had access to a tuner, so it should have been able to do the recording.