I didn't see this in any changelogs, but I traced the origin back to 1.16.

When a mob enters a minecart, they are unable to despawn, even if PersistenceRequired:0b. However, they also do not count towards the mobcap. If this is not intentional it is a large issue as it prevents players from creating mob switches, and it creates a workaround of the mobcap to summon a large number of mobs

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Video Link(Since over 10MB):

Demonstration: https://www.youtube.com/watch?v=B1HyFiHeG4c
Unedited: https://www.youtube.com/watch?v=772LkXR3hXU