MC 1.20.1 and lower, players sat in a minecart at Y .71250, but was broken in 1.20.2 snapshots and pre-releases to .46250 (fixed in MC-265296).

However, not sure exact version change happened, now if player is higher than Y0 height is 0.6500. If player is lower than Y0 height is 0.3500.

No matter Y value, players height in minecart should be .71250