Read-only essential custom field at the time of ticket editing
You set required for the custom field X of X tracker.
Set the read-only from the "authority to the field" for the custom field X of X tracker.
When you create a ticket to the new, because the default value is set in the custom field X, you can create a ticket without a problem.
Edit the ticket, when you have selected the X tracker, for the custom field A, to not set a default value, it can not be created ticket.
This is, What specification? Or would bug?