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.