Flamestrike
Legend
Quote me the rule, because all I see I'm seeing is your personal interpretation of it.
Dude, you're the one trying to argue that simply making yourself [unseen] via invisibility means I have to suddenly start guessing your square.
The rules for invisibility are clear. You gain the [invisible] condition and all the pros and cons it carries. You are not hidden but 'for the purposes of hiding you are treated as if you are in heavy obscurement' (in other words, you can attempt the Hide action pretty much at will as an action)
I can see a ton ways where the mechanics break down here as well. For example, mage casts invisibility and uses the dash action to run into a different room. By your opinion, it makes sense for someone else to simply run in the room after them on the next turn and know where to go attack.
No, the Mage cant take the dash action on the same turn the mage casts invisibility (unless he was also a Rogue 2 and had cunning action, or was a Sorcerer 3+ and cast quickened invisibility as a bonus action, and then used the dash action as his action or similar).
But otherwise yes, you are correct. A Mage can cast invisibility and move away on his turn. His opponent can follow him on his turn, ending up in an adjacent square, and then attack the mage (at disadvantage) without having to guess where the Mage is.
Turns are simultaneous - both guys are actually moving at the same time, with the dude following up the mage and swinging like mad.
After this attack, the mage is free to attempt the Hide action. He could have done it immediately after becoming invisible (should he have 2 levels of Rogue, or have cast Invisibility with quicken spell).