Particles that appear from conjured light, conjured blocks, and successfully writing iota, are temporarily not drawn when picking up an item entity.
This goes for any item entity that can fit in your inventory.
Particles that aren't drawn because of the bug reappear again as if they were always traveling in the same direction, with the same color, they do not go away.
My suspicion is that the animation that plays when you pick up an item, the item traveling in a straight line from their original point to the center of your collision hull, is related, as the particles disappear for the same length of time as the animation.

Spent a couple hours trying to fix this and I really have no idea ... my graphics chops aren't up to it.

I'm going to close this but this is a tentative close: I'd love to fix it, just not sure how.

It seems to have been fixed...
I can't tell when it was fixed, but I'm running the latest main commit, Paucal 0.1.4, and Forge 40.0.24.

On top of that, a new issue sprung from it.
These particles render through the player briefly, rather than the particles disappearing briefly. An upgrade for sure, but still a bug.
UPDATE: It renders above everything, not just players.
So instead of rendering behind everything else it renders in front of everything.

this appears NOT to occur in pure hex, as far as I can tell.