So, something in another thread made me realize something.
First, here's the definitions for Hidden and Invisible from the Expert Classes playtest:
HIDDEN [CONDITION]
While you are Hidden, you experience the following effects:
Concealed. You aren’t affected by any effect that requires its target to be seen
Surprise. If you are Hidden when you roll Initiative, you have Advantage on the roll.
Attacks Affected. Attack Rolls against you have Disadvantage, and your Attack Rolls have Advantage.
Ending the Condition. The Condition ends on you immediately after any of the following occurrences: you make a sound louder than a whisper, an enemy finds you, you make an Attack Roll, you cast a Spell with a verbal component, or you aren’t Heavily Obscured or behind any Cover.
INVISIBLE [CONDITION]
While you are Invisible, you experience the following effects:
Unseeable. You can’t be seen, so you aren’t affected by any effect that requires its target to be seen. Any equipment you are wearing or carrying also can’t be seen.
Surprise. If you are Invisible when you roll initiative, you have Advantage on the roll.
Attacks Affected. Attack Rolls against you have Disadvantage, and your Attack Rolls have Advantage.
And here's the Invisible condition as presented in UA5:
INVISIBLE [CONDITION]
While Invisible, you experience the following effects:
Concealed. You aren’t affected by any effect that requires its target to be seen.
Surprise. If you are Invisible when you roll Initiative, you have Advantage on the roll.
Attacks Affected. Attack rolls against you have Disadvantage, and your attack rolls have Advantage. If a creature can somehow see you, as with magic or Blindsight, you don’t gain this benefit against that creature.
First, there's obvious similarities in the mechanics in the EC playtest, with the main notable difference being that Invisible explicitly said you can't be seen (Unseeable), while Hidden uses the simpler Concealed.
One clause in Hidden that disappeared is the one for Ending the Condition, which got moved to the Hide action. And that is where I think the problem happened.
Basically, they merged Hidden and Invisible, because mechanically they are basically the same thing. However there were slight differences. The ending conditions for Hidden got moved to Hide, while the "can't be seen" aspect of Invisible got moved to the Invisibility spell.
Except it didn't.
Worse, for all the complaints about it in later playtests, most spells didn't show up in the playtests, Invisibility obviously being one of those we never saw.
So what happens when players complain about the incompleteness of the Invisible condition? Those complaints can be discarded, because that segment of the rule was already moved to the Invisibility spell. They just hadn't shown the Invisibility spell in a UA, so the complaints were working off of incomplete information, and could just be put in the, "They'll understand when they see the full rules" bin.
Except somewhere along the way it failed to actually get moved into the Invisibility spell. Maybe the editor forgot, or they failed to save, or it got overwritten by an older copy, or whoever handled it got fired and so the discontinuity was lost, or something.
Regardless, it's clear from the transition from EC to UA5 that stuff was shuffled around, and the unique aspects of Hidden and Invisible were supposed to be moved to the actions that created those conditions, leaving the Invisible condition with the generalized form of the common mechanics.
Thus we can see that not only is it an error, but how and when the error happened, and why it wouldn't be caught in playtesting.
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With all that said, there's still other elements of uncertainty, such as the ranger's Nature's Veil, which gives the Invisible condition until the end of the ranger's next turn. Can the ranger be seen while that's up? I honestly don't know. It's neither the Invisibility spell (though the version in Tasha's says you become magically invisible, so maybe?), nor does it use a Stealth check for a Perception DC.
It could be that if you "magically" turn invisible, that will include the "can't be seen" rider. That would be something to include in the Invisible condition itself, rather than in the Invisibility spell or other features.