TRACON Controller reported an aircraft descended below the minimum vectoring altitude while on approach.

Narrative

I was working the ZZZ Sector with some afternoon arrival traffic. I received a handoff from ZZZ1 with Aircraft X. I told them to expect the RNAV Runway XXL and descended them to 050. I then went back and cleared Aircraft X to cross ZZZZZ at or above 047; cleared ZZZ2 approach. The pilot read it back correctly. I subsequently slowed the aircraft after the clearance and read back. During my scan; I saw Aircraft X at 042 outside of ZZZZZ in a 044 MVA. I issued a low altitude alert and asked if they had the field in sight. They did so I cleared them for a visual approach to eliminate the MVA requirements; Aircraft X proceeded on the visual; ended up calling traffic ahead in sight; and followed them in landing without incident. This is a tough event to prevent a reoccurrence of from the Controller side because I verified that the read back was in fact correct; even listening to the audio after the fact. Somehow in the cockpit the restriction got lost as far as I can tell. Luckily in this instance; I observed the aircraft descending below my issued restriction quickly and ensured that VMC allowed them to be on a visual approach. The low altitude alert was definitely necessary to prevent a further descent. I will continue to scan at all times to continue to try to catch these errors in the future; and issue alerts as necessary to correct instances like this.