I've been noticing this for quite some time now and when I do my healing rotations (with or without the seal on) I lose the same amount of mana. Now I do take into consideration a paladin's mana restoration on spell casting (a long with the meta gem/trinket I have) but on sets where I proc NONE of those, I lose the same amount of mana.
I have also gone back into logs and seen how much mana I've lost and they do show no change. Is it worth ticketing or should I go to Glyph of Seal of Light?