I have a fighter character that has a base Strength score of 18. The character also was attuned to a set of Gauntlets of Ogre Power, raising his Strength score to 20 (this might be due to a separate bug, as the description of the gauntlets says it should only raise to 19). Upon reaching 6th level, I wanted to raise the fighter's base Strength score to 20, so that I could then give the gauntlets to a different party member after leveling up. When I got to the window where I could allocate my ability score increases, I was unable to increase my fighter's Strength score because the bonus Strength from the magic item was being counted in his score and 20 is the max. This interaction feels like a bug to me.
If this interaction is actually intended by the developers, I hope they reconsider. The gauntlets and other magic items with similar descriptions should not overwrite a score, but rather provide a conditional increase. After all, the description of Gauntlets of Ogre Power state that they provide no benefit if the character's Strength score is above 19, so preventing an increase while attuned to the gauntlets would make that part of the description irrelevant.