the biggest problem with the DTV is that it is something dreamed up and introduced by the MFA without consulting any other government departments. Immigration had zero input to it and that appears to be the reason they don't like and why many immigration offices refuse to extend it. The 180 days stamp is important in that 180 + extension if you can get one is less than 1 year so it keeps it very distinct from the non-immigrant long term visas. I can see them modifying the DTV in the future, perhaps even splitting into 2 distinct visas for workation and soft power with different options or removing soft power altogether as an METV is enough for most courses
that might be your problem. If you were on an extension of stay rather than an actual visa your extension of stay died the moment you cancelled your work permit. So you would have been on overstay from July
You can try asking them but they don't have to give a reason. There was obviously something about your application they didn't like. Did you apply for soft power or workation?