yes this is not pure 5e dnd this is a video game call baldur's gate 3, this information I gave you come from the official wiki and can be tested in game.
I did just test it.
Hag who has been hexed to have disadvantage on Dex Ability Checks: 50% chance to hit with Sacred Flame. Rolled 1d20 in the combat log.
Hag who has not been hexed: Also 50% chance to hit with Sacred Flame. Rolled 1d20 in the combat log.
Conclusion: Hex doesn't affect STs
Of course, this might be a bug specific to hags, or Larian might
want disadvantage to affect all 3 of STs, ability checks, and attack rolls but have not implemented it yet.
