Slow sideways movement in spectator mode is wrong at angles which aren't (close to) a multiple of 45°

When you slow yourself down in specator mode you'll notice an annoying phenomenon when trying to move sideways. You're not actually moving at the correct angle. Instead it's as if you're rubbing against an angled wall which blocks your movement on one axis. bug demo(1).mp4

This only affects spectator mode (or flying mode in general, but there's no way to change fly speed in creative), as slow movement on the ground behaves as expected with no oddities.

Reproduction Steps:
1: 42deg.mp4

• Go into spectator and set your movement speed to lowest
• Look down and have your facing at 42 degrees
• Move left
• Slightly move your mouse to increase your angle above 42 degrees.

2: repo steps.mp4

• Create a void superflat world
• Run command /execute in minecraft:overworld run tp @s -7.50 -55.00 -7.5 40.00 90.00
• Compare your movement in spectator mode at the slowest speed vs regular speed.

Observed results:
1. Your movement is very slanted until your cross 42 degrees and suddenly change direction. It's like a wall was blocking your movement and suddenly disappears.
2. You clearly move at a different angle than your actual facing so you reach a very different position

Expected results:

• Your movement would be consistent with your facing angle.