Whenever particles spawn close enough to the south or east side of a block space, they do not travel to the north or west at all. Oftentimes when spawning 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 Example

https://youtu.be/zL-mk2O7yD4