When you set a restriction on a custom user field, this works correctly. However if you then edit settings again for the activity (or section), the dropdown with the field name is not correctly set up (it's still in the 'choose a field' state.)

This applies only to custom fields. I've found the problem and it's a one-character error, but I'm going to add a Behat test anyhow that covers this situation, as it is a UI form problem and since this came up, we might as well have something to verify that the form fields are correctly re-initialised when editing in this case (also to test with custom fields in general as the existing test only uses standard fields).