When I’m duel wielding if I try to reload my secondary it falls on the floor. Happens only in Halo 3 with the Universal Reclaimer controls as far as I know.
I had the same issue when the game came out: using Recon controls.
I do not remember how it fixed ( it was a patch, I think ), but after a few months it went away.
You gotta reload the second weapon by pressing down on the left stick instead of hitting Y. My weapon doesn’t drop when I do that.