I created a new single player game on The Island. Default settings. Only mod is Fast Ammo Changer.

I spawned in a longneck and 10 of each ammo.
Simple -> Tranq works
Simple -> Shock works
Shock -> Simple works
Shock -> Tranq works
Tranq -> Simple works
Tranq -> Shock does not work. Shock is highlighted green like it is already the active ammo when Tranq is loaded.

I also tried in non-dedicated with the same results.