Answer
Making a field mandatory via the client's Properties > Mandatory option (right-click on the field) only changes it for the individual user's own personal profile, not for everyone. To make it mandatory for all users, modify the screen's Base Profile instead — that's the change that applies broadly across every user who doesn't have a personal override. If the field isn't something you're able or willing to change at the Base Profile level, an alternative is to enforce it with a Custom Event that blocks saving the record when the field is empty, which achieves the same practical outcome without touching the profile.