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
Enter creative mode
Create a 3x3 platform
Move to the south block
Move to the northern most side while remaining on the block
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
Enter creative mode
Create a 3x3 platform (if not already made)
Move to the east block
Move to the western most side while remaining on the block
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.
Affects 1.16.100
Affects 1.16.200.56 Beta