Why is FMP requiring a user to enter data in a field that is not on the layout?
Because the field's validation requires that the field not be empty. This validation option does not depend on what layout is used to edit data in the record. If there are cases where the field is allowed to be empty, remove this validation. If it must always have data, you will need to provide a way to add the needed data no matter what layout is used.
The best way to fix this depends on what needs to happen with this data. There can be many different design options that might be used in this situation. Why was this validation specified in the first place? What are the circumstances where leaving the field empty can be permitted?
Validation field options are useful as an "insurance policy" that makes 100% sure that a particular error does not take place. But the validation error messages that pop up are "clumsy" in my opinion as the message content is limited and the options presented to the user can be confusing or even options you don't want them to choose. They also allow the user to make the error and then the message appears and requires them to correct it.
A better interface design is one that, if possible, prevents the user from making a data entry error in the first place. Of course, this is not always possible. When you do need to check recent data entry for errors, a script run from the OnObjectValidate trigger allows you to provide a much more flexible and user friendly response to errors with better and less confusing options for correcting them.