Where to ask questions about specific apps?

As above - you can get to the GitHub folder for the app by clicking the 'octocat' icon on the app loader.

Even if it's not in the readme, by clicking on the app.js file and then History in the top right you should be able to get an idea of the folks who wrote it so you can cc them, for instance: https://github.com/espruino/BangleApps/commits/master/apps/ptlaunch/app.js

It's often hard/impossible to match a GitHub contributor to forum user, so from that point if view I'd say if you have something you're sure is a problem and is reproducible (even better if you have a fix!), report it on GitHub.