Chaosmancer
Legend
I've been thinking about this and...okay, bear with me, but what if the rule is structured in such a way as to favour a PC who makes the effort to conceal themselves from the enemy? Look at it this way: playing with the (vague) 2014 rules, you'd let a PC take the Hide action to duck behind a wall or something, right? And they'd make a Dexterity (Stealth) check, probably with a DC equal to the passive Perception of whomever they were hiding from. At that point, you'd rule they were unseen, and would get advantage on an attack roll, for example. Let's say the PC wants to dart (or sneak) out of cover and stab a guard, rather than shoot. How do you rule that? The actual rules don't really cover it - in theory, the instant they're out of cover, they're visible, and they don't get advantage. Which kind of defeats the purpose of sneaking up and getting into a hiding position first. The guard ought to be vulnerable in that situation, and you might rule the stealthy PC is carefully picking their moment to strike (like when the guard is looking away). Similarly, if they wanted to sneak past the guard, would you make them take the Hide action every turn they were tiptoeing away? I think most of us would let the initial Stealth check result ride.
So, let's take that (common) ruling and turn it into a specific rule. What does that look like? Well, maybe it looks like a PC becoming "Invisible" when they succeed on a check, and gaining the benefits thereof until the situation changes radically. Either they make an attack (so the condition ends anyway), or they sneak away and go unnoticed...in which case, yeah, they'd stay Invisible as long as they don't make a loud noise. If they step right in front of someone? Well, they'd get advantage on their attack roll, which is not unreasonable in that circumstance. Or, if they don't attack, you'd rule that the spotter would see them automatically on their turn, ending the condition anyway. They can only run away so fast without making "a noise louder than a whisper", so the worst case scenario is that the Invisible PC gets 60 feet away before the guarded who spotted them can react, automatically spot them and end the Invisible condition. Remember that Surprise as it was doesn't exist any more: there's no free turn, just advantage on your Initiative roll if you're Invisible. So that extra turn to run away, or get the drop on an enemy, is instead built into Invisibility as a condition.
What I'm saying is, this rule seems to be set up to essentially give a PC the benefit of the doubt when it comes to stealth. That might be why it's a DC 15 check instead of DC 10. They used an action to Hide and made a successful non-trivial ability check (or alternatively burned a level 2 spell slot); let them have a bit of Invisibility, as a treat. It's still highly conditional, so at most you might get one or two slightly weird interactions, which are fairly easy to rationalise. "How can I be Invisible when he's looking right at me?" "Well, you stepped out of the shadows; he didn't know you were there until that moment, that's why you got advantage on the attack roll. Remember, this is all happening in less than 6 seconds!"
Yes, I believe that is 100% what happened and what the intent is of this. It is about good faith play not being technically over-written by the rules.