Might be related to scopes or sights on your weapons - if one is scoped and the other not, try Disguising with the scoped weapon selected - if it zooms in, you're retaining your selected weapon's aiming. Fix? swap weapons. If not, try the other way around and see - it might use the non-selected weapon's scope option instead - obviously in that case the fix would be to have the scoped weapon selected when using Disguise.
Let us know what you find, could be useful information to know.