Have also experienced the bug where the arty gun cannot be reloaded, as recent as yesterday. On PC.
Arty engi built arty gun was in use (Arty support gunner, arty engi reloader). Both left the gun for a short period, this included some deaths. Arty gun was unable to be reloaded again, it does the reload animation, doesn't load, there are rounds available. Same squad, same gun, different gunner (arty SL), same reloader (arty engi). Arty SL also tried reloading, wouldn't work.
We were going to try dismantling and rebuilding but didn't get the opp, or to do further testing. Would be interesting if it worked with the same arty support gunner.
On another occasion, different match/day, an arty player from the other arty squad tried to use the HQ built static arty gun, and they couldn't reload. I had built the arty gun, but switched to SPA, swapped back to the same static arty gun and I could reload and shoot. The other arty squad player could then jump on the gun and reload for me.
There's another bug, where the reloader can infinitely load the arty gun after its been loaded, this prevents the gunner from shooting, doesn't use a round since there's one in the gun, but it's a pain. This is separate to the first issue.