I've received some .mcc Closed Caption files that were exported from MacCaption and no matter what I do they only come in as 720x486 @ 24FPS. I've tried modifying them to be CEA-708 Service 1 and set the TC to 23.976 but nothing I do sticks.
Now of course the problem is that any export I do only has the SD CC in it. I'm not sure what to do as this has always worked properly in the past.
As I don't have access to your system nor know your settings, I can only use my experiences as reference.
The mcc caption file will come in a 704x480. It is dependent on who you get the file from, but our cc files have the option to receive from our captioner as drop/non-drop frames. It sounds like you may need to modify direct from Mac Caption.
We receive both 608/708 in the mcc file, but you have to make sure your display is set to show 708's/608's in the program monitor (click the wrench, enable CC and change preview settings to 608 or 708)
Also make sure v1 and v2 on the timeline are enabled to receive both captions. (If only one is enabled, only 1 will show)
Hope this helps
Production Editor and Motion Graphics, PBS Charlotte