(Minecart Improvements) Minecarts on a downwards slope jitter/shake under some conditions

Note: this bug only occurs with the 'Minecart Improvements' experiment enabled, regardless of the value of max_minecart_speed and whether or not the minecart is occupied.

When fences or walls are placed beneath the supporting blocks of a sloped rail, minecarts travelling down the slope shake and decelerate unexpectedly.

Steps to reproduce the issue:

1. Build rails on a slope

2. Place fences/walls below the blocks supporting the rails

3. Ride/send a minecart down the sloped rails

Expected Result:

The minecart travels down the slope smoothly.

Actual Result:

The minecart shakes as it descends, slowing dramatically.

Attached:

• Comparison of speeds between shaking minecart (actual) and normal minecart (expected)

• Player POV if they are riding the minecart.

Both videos recorded with gamerule max_minecart_speed set to 32.