I have also experienced this issue multiple times. From my experience over the last 6 months, I’ve found this:
The glitch applies to all types of Ground Wars projectiles.
The projectiles don’t necessarily freeze, sometimes they just move very slowly.
They are client-side. I have had projectiles stay still and spinning across multiple games of Ground Wars, and one time I even saw one in a game of The Bridge after I had been playing Ground Wars.
The projectiles do not interact with any players or blocks.
However, I’m still unsure about what causes the glitch.