As the heading says, particles get summoned when you are in Spectatormode and "run" through blocks. See screenshot
Steps to reproduce:
1 Make a world that has enough blocks to fly through without any gaps. The superflat template "Tunnelers' Dream" is recommended.
2 Run the command /gamemode 3
3 Fly through the blocks while pressing the "Run-Button"
(in Default Setting it's [Ctrl])
4 Try to look behind you while flying
Linked issues
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I believe this has been reported before, but I cannot find the report 😞.