These two fields are both text fields but numbers are entered in these fields.
Make sure the "result type" of the calculation field is set to "Text".
If this is an auto-enter calculation added to a table that already has records, the calculation will evaluate only when you modify data in either of these fields. Change the field type to calculation and your existing records will update to show concatenated text.
If you don't want this to be a calculation field, you can return to manage database and turn the field back into a text field with an auto-enter calculation.