Have you deactivated this option after you used it to remove the level requirement for the specific item?
Try to uncheck it but they still immune to any damage. Only after go to menu and re-join to game I can kill them.
Which quest are you stuck on?
Sorry for late reply. Item levels are shown on right corner of any weapon/item, for example if you are level 16 character you will drop weapons/items in level range of 14-16. All stats of said items/weapons scale with it's level. So if you dropped let's say Queens Call legendary pistol on level 25 and them scale it to level 40 using cheat - all stats are going up to match new level.
So item scaling option (you can see it in Gibbed Editor for BL2) allows you to scale all items in your inventory to your current level.
So if you got on level 16 a great weapon that has level 16 too - you can scale it to your current level (let's say 35) to keep using it instead of ditching since even white weapon on level 35 will be better than that.
So item scaling allow for constant item relevancy. Also if you start new character and have bunch of level 50 legendaries- you can scale them to level 1 and make your new character use them too.
It's very likely caused by the game, by design. I expect reducing the item level requirements also reduces the overall damage output, no matter what the tooltip says. Instead of removing the item level requirement (or item level) it should probably be set to your current level instead.