These particles are just hanging in the air, not disappearing. They are still animating, but are not moving.

How to reproduce:

1. pass through a naturally generated gateway into a dimensional dungeon.

2. exit the dungeon. the gateway is now a rift.

3. place an iron dimensional door on the rift

Reloading the world makes the particles go away.

Graphics mods present: optifine HD_U_D1, better fps 1.4.8, no potion glint 1.0.0

Update:

Issue persists even when no graphics mods are present (I am using cubic chunks though, forgot to mention that last post). Additionally, the issue also affects rifts without doors.

The particles should slowly disappear. Are you sure they don't? They can take up to a few minutes to fully disappear?

I watched for quite some time. I can watch for longer, but I don't think they decay.

After watching for a few minutes, I can confirm that they do decay eventually. It still seems a little long though.

Yes, that's what's intended. The particles represent void particles that escaped from the other side of the rift, not just any regular minecraft particle.

Fascinating. Perhaps there should be some use for this. Surely void particles must have at least some practical applications.