Just got passports for my kids recently. My 14-year old was scowling in his photo. They accepted it.

They know that nobody standing in front of a customs officer that is looking at their photo will actually be smiling. So having it neutral or even scowling as your experience will be much more likely to match. /s

Some countries (definitely France) are implementing automatic passport photo checks. Those checks are not at all rigorous concerning smiles. I managed to get a photo I'm actually proud of in my new ID!

They do have problems with facial hair, though, IME.

But but but - I have resting happy face!