It's unclear exactly what parts are failing and why, as far as I know - just that sneak attack as a whole is often failing to properly acknowledge when its requirements are met.
The issue with it not granting sneak attack when there was no advantage but a suitable creature hostile to the target close to them is not new - it's been doing that for the whole EA - it's just more visible now that it will outright deny you the ability to use the 'skill' when it thinks you can't, as opposed to previously where it just let you attack and treated it like a normal attack.
I have a *hunch*, though I've not got sufficient data to prove it, that the error is actually in the order of circumstance checking in combat. I *suspect* that in situations where you have both disadvantage and advantage, (such as a high ground plus long range, being the most common example), which cancels out and gives a straight attack roll, AND an ally close to your target, it still denies you sneak attack because it reads a disadvantage situation *first* and locks it out right away; this would be incorrect coding of the ability, if that's what's happening.
