Flamestrike
Legend
I'm the OP of the thread, my goodness this has started quite the discussion!
So listening to all of the arguments so far, I think its quite clear that the monk in my situation should not automatically be hidden after he attacks and moves 100 ft. Aka he has to do "SOMETHING" to be undetected even while invisible.
So then it comes down to how much is "acceptable". Obviously there is always the full hide action, which is a significant investment to maintain your stealth. Perhaps I could consider a bonus action when invisible....it still requires some action (and in the monk's case that is not a trivial investment)...but its not the full degree. This would represent being stealthy while invisible still requires effort....but it is easy easier than regular stealth in terms of time investment.
The Hide action is a single action and you remain hidden till you reveal yourself.
You can move, search, cast some spells, and do anything else in subsequent rounds other than attack or do something else (yell something out etc) that reveals you.
In order to be found while hidden your opponent needs to take the Search action in subsequent rounds to find you (presuming he doesn't reveal himself somehow). When they succeed, youre found.
The action economy is preserved and it all balances out.
Notably Rogues of 2nd level or higher who are invisible can attack and then totally dissapear via the Hide action as a bonus action every single round.
With expertise in Stealth, they're all but assured of this each round.
Action surging fighters can Attack then Hide. Sorcerers can quicken spells and Hide in the same turn as well. High level rangers can so the same with Vanish.
Your monk can Attack, then move away, then Dash and then Hide. It'll just take him 2 turns to do it instead of 1.
Its a question of action economy only.