The Bug
If a dispenser attempts to dispense a minecart when there is a minecart one block in front of it, the placement fails, making the dispenser drop the minecart as an item. However, unlike the dispenser, a player can place a minecart next to another one.
This does not affect worlds with the "Minecart Improvements" experiment disabled.
Steps to Reproduce
See the attachment titled Setup.
Ensure you are on a world with the "Minecart Improvements" experiment enabled.
Place a rail and a minecart on top of that rail.
Place another rail next to the existing rail, with a dispenser facing the unoccupied rail. Place a button next to the dispenser.
Place a minecart on the unoccupied rail yourself and note how the placement succeeds, which is the expected behavior.
Remove the recently placed minecart and load the dispenser with one.
Press the button to eject the minecart onto the rail.
Observed Behavior
The minecart placement fails, and gets ejected as an item, despite there being no minecart in its position and the player being able to place the minecart in the same spot successfully.
Expected Results
The dispenser would successfully place the minecart onto the rail.
Can confirm: