I opened up my Git-stored PowerApp and realised that the code was out of date - the live published version has my latest updates, but not the version that opens in the studio..
I can only assume that when I last saved and published the app, it didn't commit the changes to the Git repo - evidently a bug.
To fix it, I had the idea of cloning the repo, updating it manually with the changes, and making a new commit. I therefore tried exporting the app as a zip, but unfortunately the resulting archive doesn't match the structure in the Git repo, preventing a simple replacement of the files in the repo. Does anyone have any ideas?