Possibly a special bonus, but most likely an error.
I have looked through the Monster Manual, and have found nothing to explain why the Night Hag has a +1 to each of those skills, aside from this text
A skill bonus is the sum of a monster's relevant ability modifier and
its proficiency bonus, which is determined by the monster's challenge
rating (as shown in the Proficiency Bonus by Challenge Rating table).
Other modifiers might apply. For instance, a monster might have a larger-than-expected bonus (usually double its proficiency bonus)
to account for its heightened expertise.
It is notable that when an entry in the manual has 'double its proficiency bonus', the only indication is the number itself, so it is possible that there is an unspecified bonus present on the Night Hag.
In addition, of the 3 Hags present within the Monster Manual (Sea, Green, Night), the Night Hag is the only one that has items (Heartstone, Soul Bag). While the Heartstone does list an effect that benefits the Hag, the bag does not, only stating that it captures the soul of an evil humanoid that dies due to the Night Hag's Nightmare Haunting action.
So, given that all of the Night Hag's listed skills have a +1, there is a slight possibility that it is due to a bonus which is unlisted, most likely due to one or both of the items it can have.
However, because nothing in the description nor stat block of the Night Hag mentions that anything on or of it produces a definite bonus to any skills, it is likely an error.
As to reporting an error, there are 2 options: 1- make a post in the bug report forum on DNDBeyond; 2- try contacting Wizards directly.
As for contacting Wizards directly, there are 2 things I found, the first is a customer service number - 1 (800) 324-6496 ; the second is this web page: https://dnd-support.wizards.com/hc/en-us
Be aware however, the DNDBeyond bug report forum likely only applies to the site itself, so if you may have to contact Wizards directly if you consider this important.
