Obviously the tomes and manuals are clear-cut in how they function, but when it comes to magic items that "overwrite" or "replace" an ability score with a value, such as gauntlets of ogre power, what rule prevents you from using an ASI to increase your strength score from 19 to 20? People online talk about it only increasing your "base" strength score, but where is that distinction made? Your strength score is 19 when you wear them, is it not?
When researching this online, everybody always seems to operate on the implication that there is some distinction made between your "base" scores and "temporary" scores granted by an item, but for the life of me I couldn't find any actual citations or rules in the 5E books that actually solidify this distinction. The only thing I could find was this sage advice that runs seemingly completely counter to that overarching assumption.
So, my question is: how do magic items that adjust ability scores actually interact with ability score improvements? Preferably with some form of citation that backs up the explanation.
