If you destroy a mob spawner that is configured to spawn Illager Beasts or Pillagers (which currently cannot spawn naturally), destroying it under certain circumstances will cause the flame particles inside to remain visible even after the spawner is destroyed.
To reproduce, use the following procedure. Please note that it may take multiple attempts to do so, due to the behavior of spawners.
Go in to a world with Cheats enabled, and make sure you are in Creative Mode.
Use the /give command to give yourself a spawner, and place it.
Use a Pillager or Illager Beast Spawn Egg on the spawner you just placed.
Stand back from the spawner and wait for the particles inside to increase. (Basically, wait until it attempts to spawn a Pillager or Illager Beast, but fails because they cannot currently spawn naturally).
Break the spawner. The particles will be left behind, and they will persist until logout.
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It's not just the new Mobs. I am experiencing the same problem with Ocelot and Pig Spawners. The particles don't disappear while the spawner is active either. New Particles keep appearing though, so the overall number of particles grows to a ridicilous level. Again Particles stay even after breaking the spawner and replacing it with other blocks. See screenshots attached.
UPDATE: Added two additional screenshots.