Accidentally discovered during v2.8.1 testing, though the issue dates back at least as far as v2.3.0 (and, probably, to the beginning). The "Set as Default" button does not work (page reloads but no default is set) if:
One/any of the controlled values in that list are already suppressed, and
The enumeration list is an uneditable list (e.g. the bug can be seen in `subject_term_type` but not in `subject_source`.
Any combination of suppression and setting defaults under Manage Controlled Values makes the default setting unchangeable.
So, if you:
1. Set a default
2. Suppress some other thing on that uneditable enumeration value list
3. Attempt to unset the default set in step 1
4. The default can't be unset and remains the default (until you unsuppress the thing(s) that were suppressed in step 2)
1. Suppress something(s)
2. Attempt to set some enumeration value as a default
3. The default won't set (until you unsuppress the value suppressed in step 1)