I've tested this in a new creative world created in 1.16.1, for example when I spawn a husk using the spawn egg, while being high enough in the world so other hostile mobs have despawned, I see a mob count of 1 (I used the Fabric loader, Fabric API and Carpet mod to see the mobcount), when I get the husk in a minecart, the mobcount is back to 0, when I break the minecart the mobcount is 1 again.

To check it is not the Carpet mod count is the culprit, I tested my upgraded 1.15.2 world with a mobswitch, which has 640 shulkers in minecarts, these were not counted in the mobcount of Carpet, and there were normal hostile mobs spawning around me.