I'm personally of the opinion that the description needs revising. My line of thought is that since the description says "Lose 3 health per level, but gain 1 mana per revealed tile", if I cast the glyph on a completely revealed map then since no tiles are revealed, no mana should be gained.
I'm not sure I see a flaw in my reasoning? The only alternate reading I can see is gain 1 mana per already revealed tile, which would be absurd.