It's not a bug, you started the reload process but cancelled it.
Your mag indicator isn't even showing a full mag, it's hollow when this happens.
Empty the chamber and reload, then sprint or cancel by jumping/vaulting/swapping firearm, you'll see its not a bug but a feature.
It's very frustrating for it to happen, it's happened to me many times, tac reload - cancel without thinking about it cus you can't sprint and reload (Why the hell not? We don't need hands to run as we're bipedal.) and go to engage a target only to hear CLICK, or in some cases there is a round in the chamber and then you die.
It's not being cancelled. I've watched very carefully when I think it'll happen and sometimes the ammo goes up and snaps back down when I fire and other times it just doesn't go up and the indicator fades out. I am absolutely sure that it is a glitch. Several times while waiting for start I'll get a glitch where the speed reload animation plays, but the mag-out and mag-in actions play in sync with the tac reload. MP is especially glitchy.