Hi,
If you are sure that the form field is correctly mapped to this custom field, please check the data type of the field.

And make sure that the type of value you fill in on the Marketing Form matches the data type of the field.

There are also some values that can be filled in on the Marketing Form, but there are restrictions when entering the field directly.

If all of the above items are okay, please check whether Field Security is set for the field.
If this helped you, I'd appreciate it if you'd mark this as a Verified Answer, which may in turn help others as well.
Best Regards,
Nya