I have found that if you switch weapons, then switch back to the original weapon, the game is automatically switching your ammo to the base type. In some cases, not having any of the base type leaves you with an empty weapon.

For example, I have a modded hunting shotgun. I use slugs with it exclusively. If I switch to my sniper rifle, then back, I have an empty shotgun, or some regular 12G if any is in my possession. This has led to me regularly breaking down all base ammo and always selecting ammo type when changing weapons. I still get caught our regularly and forget to do this.
It has also resulted in deaths in close range combat when pulling out an empty shotgun or one with inferior ammo.

Frankly, the game shouldn't be arbitrarily reloading your weapons with the base type.

I have noticed this with multiple weapons/ammo types (308/357/10mm/12G)

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EDIT: Sorry. Moved to bugs thread. Moderator please remove.