For example. I have a bar stall and i have to assign someone to it every damn time i want to use it. Also for example, i assign someone to a scrapper and they never stay at it. Am i doing something wrong or is this just a silly settler thing.

They stay assigned in my game, but make sure you don't try to assign any named NPC to any store...or anything much really.

Happens to me a lot too. Seem to be happening mostly after the settlement has been under attack.

They often dont stay at their assigned position but they are assigned and the job gets done magically. Sometimes you can even talk to your shop worker while they're standing on the roof of a brahmin pen and get the option to buy and sell. true test is if the shop has a red or green worker icon.