I have now deleted the resize image add-on, removed the automatic styling, and synchronized all my devices.
In fact, I removed all the styling associated with the note type. This causes the text on Windows and AnkiDroid to be left-aligned (as expected), but there is no change at all in the styling on AnkiMobile (text is still center-aligned, and there’s still the unwanted line break), indicating that the changed styling doesn’t propagate to my iPad.
However, when I create a new deck and move cards around (on Windows) and synchronize, the changes show up on AnkiMobile (as expected), so the sync isn’t completely broken. Looks like it only affects the styling or something?
Closing AnkiMobile, restarting my iPad, forcing a one-way sync, checking empty cards, rebuilding database, etc. doesn’t seem to fix it.