Theotar Bug or?

Hey
i run into an odd interaction with Babaric Sorceress and Theotar!
Opponent played Babaric and swaped my 6 Mana Spell into 9 Mana.
Played Theotar the next turn and swaped my 9 Mana Spell with him.
Occasionally in his next turn he could play the Card for 0 Mana and his 6 Mana swaped spell on Turn 6. Is there an Interaction i didnt know or a common Bug?
And no there wasnt a Kalecgos on board ;p