TRACON Controller reported an aircraft on vectors to avoid special use airspace descended below their assigned altitude and below the Minimum Vectoring Altitude.

Narrative

MEDVAC was at 9;000 ft. in the downwind for LDA Runway XX. Was given a 090 heading. Was then turned to a heading of 070 to miss R-XXXX; active below 9;000 ft; and a descent to 8;000 ft. MEDVAC was turned to join XX LDA. I issued the clearance right as the aircraft was at 8;100 ft. MEDVAC said they have 7;000 ft. but will stop at 8;000 ft. and read back the rest of approach clearance. The aircraft advised they would stop at 8;000 ft. so no further action was taken other than to confirm the altitude was 8;000 ft. The MEDVAC got down to 7;600 ft. right prior to starting a climb back to 8;000 ft. 1.5 miles prior to entering a 6;500 ft. MVA. No SA (Safety Alert) was given as the aircraft was not in an unsafe proximity to the terrain and had already been advised of the appropriate altitude and was taking action before descending below the 8;000 ft. MVAThought this was clearly a pilot error. Extra headings were given to miss R-XXXX. This in the past was always active to below 8;000 ft. This worked for years. They never needed 9;000 ft. before but always seem to take it now for no apparent reason. This is right under the finals for ZZZ Airport and we lose a usable altitude to accommodate for what seems a pointless necessity. Range Control should only used what they need and not take precious airspace just because they can. Ideally this restricted area should be completely re-evaluated. There was never a proper impact study done and it is in a dangerous area for live fire military activities. Everything they do can be accomplished in the western portion of the Restricted area.