To be honest, pets not receiving mana from the tunnels is a sensible business move, thus, I put to you that this is likely something they have chosen to do.
Why would anyone buy those mana boostery things, if you could simply have 18 minutes of virtually completely unlimited mana?
Before you reply, saying how wrong I am, I'd advise you to think about this:
When you are on a pet, you are a character, yes?
Then, why do all other characters gain mana from the tunnels?
If this was a logic failure of the server/client, then, you'd likely find that other races don't gain mana, or that no character does, or even that some pets DO gain mana.
Thus, the server/client is/are actively working to keep that character from gaining mana.
I'd open up that EU pleione if I were you, and see if you can find out if the client has any input in regards to that, such as some kind of IsPetCharacter function, I'd doubt it, but Nexon makes stupid mistakes all the time.
Of course, that was all speculation, but, last I checked, Nexon was in this to make money from us, and game mechanics were their tools in doing so.
Nao stones anyone?