When I enter a minecart, the viewpoint is considerably high. I have done extensive tests and reached a 100% reproducible scenario.
Place a minecart, simultaneously start a timer. Enter it.
Notice the camera position is very high.
4-5 seconds in, it is lowered slightly.
exactly one minute after the minecart was placed, the view point is shifted to where it should be.
4-5 seconds after that, it again lowers very slightly.
It then enters a loop of being reset every whole minute and lowering slightly 4-5 seconds after that.
Why does this matter? If you place a powered rail against a wall, and place a block with a button one block away so that it would activate the powered rail (see image), you can't use the button until the first minute is over (image includes backup button). This breaks the simplest of designs.
Tested in multiplayer survival and in singleplayer creative (the latter in multiple worlds).
My first search for the issue yielded nothing, but later I found
MC-19967. It lead me to find out that this problem is averted if the minecart is placed on a diagonal track. MC-19967 is apparently related but is a different issue.