Thanks @NateWr and @marc,
I didn’t find any related post to this issue and that’s why I’ve created a new one. Now I see it’s been a topic for a while. Sorry for that.
Anyways, following what @NateWr says (and also @marc here for 3.1.2)
the UI requires it if the user’s focus enters the field.
I see that the UI requires these fields even if the user’s focus doesn’t enters the field. So I don’t find any workaround except filling in both fields.