Happens at caches, too. I have to switch to nade launcher, tap F and I'm good to go.
Edit: only happens for Light Carrier
That's because you only have room for 1 grenade with the light carrier. Switching to heavy carrier will allow you to carry more grenades.
I will check again with the latest update.
The problem is that my gun is not pre-loaded with another round when using a light carrier.