When you have a companion they at least chose their own preferred stuff in their inventory in some previous Fallout games. And since it is also possible to give settlers inventory, they probably also do.

But it's not so fun to find every single settler to keep giving them ammo for example after every settlement raid, so it is kind of a bug, because the game is supposed to be funny or entertaining.

So this is a suggestion to a second settlement workbench called "Guns & armor" something for settlers, so they only equip what ever is stuffed there and not in the shared workbench!

It'd also be nice if you could put a computer terminal next to the Guns & Armour bench where it'd list all the people in your settlement and you could manage their gear in a similar fashion to Fallout Shelter.