Going through customs, in most countries their policies allow them to search, image, or confiscate your phone, but not evilmaid it or rubberhose you. For some travelers, that's their threat model.
I find it hard to believe they would be able to compel you to unlock your phone but not compel you to unlock an individual app.
I find it hard to believe they would be able to compel you to unlock your phone but not compel you to unlock an individual app.