Over on the Dogsey forum, people are commenting on the increased density of dogs' coats this year, because of the very cold winter ~ so perhaps it's the same for cats?
And the behaviour you describe at the vets i.e. keeping still while the vet nurse detangles him, but having a hissy fit if you try the same ~ is not uncommon
When confronted by something scary, frightening or novel, animals have different behavioural coping strategies ~ "the 4 Fs" ~ Flight, Fight, Freeze, Fiddle about a bit.
The first 2 are easily recognisable & flight is the method of choice for most cats, preferably upwards! Fight is Merlin's option when you try to detangle the matts, probably because flight isn't possible if he's restrained ~ so he chooses fight, partly because he's not scared of you so using "fight" isn't going to result in an injury to him, & probably because it's worked before ~ you let him go & stop the detangling
However the vet nurse is an unknown quantity & being at the vet surgery is a pretty scary thing as he's out of his comfort zone so Merlin will not be feeling confident. Flight isn't an option if he's restrained, fight is not a wise move as he doesn't know where to go to escape & might end up worse off, so freeze is the only option open to him. So what you're seeing is not really a well-behaved cat who is being "good" for the nurse, but a cat that is too frightened to dole out the same treatment that he gives you! So he sits there & puts up with the detangling.
My cats behave in exactly the same way at the vets!
The 4th "F" ~ "fiddle about a bit" ~ is what cats do when they're unsure, not confident, feeling a bit hesitant/wary, but not so scared they need to run, fight or freeze. You often see this when a cat walks into a room & is not sure of the reception it will get, so it walks into the doorway, sits, has a quick wash, walks a bit further, has a scratch, then meows, then comes further into the room etc. Often just a quick greeting back, calling the cat & encouraging it towards you, allays any hesitancy.