What do you do to make it come back?

You just have to put retry in console. hud_reloadscheme doesn't work ;/.

I asked Flatline about this before.

Are you Bona in an alt account?

You can just rejoin the server or type 'retry' in console.

What?

I had the same problem, unbinding f (inspect player/weapon) fixed it for me. When, say I inspected any skin or player in game, huge window of what this person is carrying popped up and my ammo was disappearing even tho I pressed f again.
If it's not inspect panel, then you probably did something wrong in clientscheme. Switch your hud to a different one - see if problem is gone.
Didn't read

I asked Bona what he usually does to make it come back so that I have a clue of what's going on, and then you answered, I just got confused on why you were responding...

same problem here lol. i just leave the server and come back and its fixed.
its a tf2 bug probably will be fixed soon.

This also happens with objective icons such as cl or pl

Usually I restart tf2 entirely because just retrying doesn't always fix it for me, I was hoping it would go away but now it's really getting in the way

THIS IS HOW AMMO BREAKS:
1. You press whatever bind you have set to inspect and you bring up the inspect panel
2. You press tab, which doesn't properly reset the inspect panel.
3. Ammo is gone.

THIS IS HOW TO FIX YOUR AMMO:
1. Ammo is gone.
2. You press whatever bind you have set to inspect and you bring up the inspect panel.
3. You either shoot or press your inspect bind again while not looking at a player.
4. Ammo is back.

please that is so dumb.

ok? I didn't know what the problem was so I just fixed it temporarily by doing that, thanks for telling me how to fix it, but you didn't have to be a douche about it

it wasn't a personal attack bro

This happens because you inspect someone. Inspect just a wall (Usually the default command is "F") and it should come back.