Is it possible to be hit by a spell cast by a mage who is in a TimeStop turn, while my mage is currently invisible? Is this a bug or is it actually foreseen by the rules of the game?
It is absolutely possible, no bug. The rules ( see here) may not seem super crystal clear, but after it happened to me to be the target of a spell while I was invisilble and the opponent used time stop, I asked clarifications to the game "janitors" and it is applied as intended (no bug).
During a Time Stop your opponent is not covered by any enchantment or other spells as you hit from a parallel time line.
However, I don't know what would happen if both mages cast a time stop on the same turn.