This might be a duplicate but I can't find it.
To be clear this has nothing to do with Halfling Luck trait. Or similar abilities that allow the reroll of a die that rolls a natural 1. (I was apparently incorrect though.)
This has to do with a situation such as the Path of the Zealot barbarian's Fanatical Focus feature (Xanathar's Guide to Everything, p. 11), which lets you reroll a save that you fail:
Starting at 6th level, the divine power that fuels your rage can protect you. If you fail a saving throw while you’re raging, you can reroll it, and you must use the new roll. You can use this ability only once per rage.
Situation: Barbarian fails his save and is afflicted with bestow curse. Now he has disadvantage on Constitution checks and saves. He is targeted with disintegrate and fails his save. Using Fanatical Focus, he chooses to reroll that save. He would obviously have disadvantage again since the curse is ongoing. I think we could all agree to that.
However, what if the disadvantage or advantage was only imposed for that effect? Such as a Sorcerer using the Heightened Spell metamagic option:
When you cast a spell that forces a creature to make a saving throw to resist its effects, you can spend 3 sorcery points to give one target of the spell disadvantage on its first saving throw made against the spell.
The ability says you "reroll that save", therefore I argue that you would have disadvantage again as it would still be the first saving throw against the spell albeit a meta-game solutioning of that resolution.
I was unable to find anything on Sageadvice.eu although it is getting harder to sift that data. And I did not find anything in the errata or the Sage Advice Compendium (2017).
Is there anything to support this ruling in the rules that I missed?
This seems intuitive and intended however I see nothing that alludes to it.