Aaaaah, thanks sdetwell! I ah, just solved it, and the answer was very silly.
For any future newbs having this issue across multiple modules, check that the files for modules you are referencing are actually in the Magic Mirror’s modules folder. A few of mine - MMM-MovieListings and MMM-MyCommute somehow got installed in ~/MagicMirror and not ~/MagicMirror/modules , which explains why it was happening in both places. Whoops! MyCommute is now working perfectly.