The bug

Various particles will stop when spawning on the south or east block boundary, sort of like hitting an invisible wall, not travelling north or west at all. Oftentimes when particles spawn on the east, they also have too much speed to the south.

Steps to Reproduce on the south

1. Enter creative mode
2. Create a 3x3 platform
3. Move to the south block
4. Move to the northern most side while remaining on the block
5. Fly up and fall

Here, the particles will appear in a line, never travelling to the north at all

Steps to Reproduce on the east

1. Enter creative mode
2. Create a 3x3 platform (if not already made)
3. Move to the east block
4. Move to the western most side while remaining on the block
5. Fly up and fall

Here, the particles will appear in a line, never travelling to the west at all. Oftentimes, these particles in particular travel to the south in a very quick manner.

Video

particles.mp4
https://youtu.be/zL-mk2O7yD4