When launched in PIE mode (playing in the editor window itself), the config file used is Saved/Config/MacEditor/GameUserSettings.ini.
Changing the resolution and fullscreen mode has no effect but the data is loaded from this file and written to it when LoadSettings and SaveSettings are used.
When launched in Standalone mode, the config file used is Engine/Binaries/Mac/PIEGameUserSettings0 (note the lack of extension). As it’s part of the engine, it should not be manually edited for projects.
I hope this will be useful to whoever runs into the same issue.