Lucio’s projectiles will stop after traveling for a very short distance from the original spot they were shot from and become visual obstructions that hover in one spot. Due to the burst rounds of projectiles, they tend to clump together in one spot.
Both ally and enemy Lucio projectiles with perform this behaviour. The enemy Lucio projectiles tend to be more visually obstructive.
I’ve already reported this two months ago. Graphical Glitches (Residual Artifacts) from Projectiles
It’s been around for at least two seasons, if not three. At one point it seems they did fix it for a brief while after my report, or at least tried to, and the duration of the linger shortened significantly, but I noticed it’s back again this season. I couldn’t post a reply directly to my own original thread without someone else replying before me, so I just edited it with an update, but not sure if they actually see that. At the time I saw, it looked like the bug was manifested for Symmetra right clicks, but now I’ve seen it show up for Genji and Lucio’s projectiles again too. So the bug is back full force.
Please make a reply in my linked thread as the information there describes the bug in full, including the specific conditions for replication. It doesn’t actually happen in every normal match.
I didn’t know it was an ongoing bug! I just saw it happening and wanted to report it! Personally, I don’t like this system of having bug reports as forum posts instead of tickets to send directly to Blizzard. It’s stopped doing it for Lucio at the moment…but it’s doing the same thing on Moira’s orbs now instead 
such is game development, I suppose.