The bug
When I get in a minecart and look around, it feels off compared to looking around when not in a minecart.
Linked issues
Comments 2
This actually appears to be a duplicate of MC-206540, since this ticket provides more detail about the issue and has a Mojang Priority attached to it.
Can confirm in 20w51a. Whilst in a minecart, your mouse sensitivity is seemingly reduced by a small amount. This is a very subtle issue and may be quite difficult to reproduce as it is barely recognizable.