Well he can be. A DII is an equivalent rank to a SGT I. DII is the first supervisory Detective Rank. If you were a Sergeant before making Detective II, you can go back and forth between Sergeant I and Detective II at will. However, if you were never a Sergeant, you cannot go from Detective II to Sergeant. What you can do is downgrade. Downgrades are rare but it happens. I had a friend who got fed up being a Sergeant and he basically asked to be downgraded to P3 and kicked back to a Patrol Car. They obliged him. The DII in the show probably felt like he wanted to go back to Patrol so he downgraded to his last Patrol rank and went back to Patrol as a P3. It happens.
Hey, since you're here, can you do me a favor and decipher Lethal Weapon for me?
I mean, those guys are "sergeants" but Riggs wears plainclothes and Murtaugh wears a suit. They do detective work but seem to be specialized in nothing and chase everything that comes their way.
Then they get busted to patrolman, while Riggs wears a ponytail. Then they get reinstated as sergeants, but then get promoted to Captain by "Chief's prerogative". Then they go out as Captains, dressed the same way, and still do detective work, instead of being division commanders or whatever it is that captains do.
Can you just tell me that they have zero accuracy and be done with it?
My S-2 buddy just about had a fit everytime he tried to explain Lethal Weapon's rank system. BTW, he did what you stated above, kind of. He was a P-3 who went to S-1, then voluntarily went down to a D-1 because he wanted detective experience but for some reason did not go D-2 (maybe there were no available slots?) but went back to S-1 field supervisor and then S-2.