lol how did this happen?

Right, so I did test this. What you do is when you go restock on ammo, you actually unequip your current grenades. That way, when you land on the incendiary, the game treats it as you already knowing how to equip and unequip and then auto equips it. However, you NEED to unequip the grenades or else you'll still get the dialogue box and have to switch over to incendiary.