Ubuntu 20.04, JabRef 5.1
The following image shows the dialog to customize entry types. I want to delete the entry "Patent" because I never use it. I click on the trash icon. "Patent" disappears. I accept changes. I open the entry type dialog again.

Result:
"Patent" appears again

Expected behavior:
"Patent" is deleted

Alternative behavior:
"Patent" is a standard field in bixtex, the trash icon shouldn't only appear for user defined entry types.