There's flow, being in the zone, concentrating. But there's also playing automatically. I would be concerned about playing automatically and not being in control. Not that you're always in control of everything, or that it's easy to just regain control. But if you're playing automatically and something happens, maybe you're startled or you hands forget the next part, then you're stuck. In that case, I would lean toward slower playing, finding some memory spots just in case you need to restart or have something to focus on, and getting your mind back into the piece (very slow playing, analyzing it again). That way it's less automatic and more controlled.
I remember thinking once while something like that happened, "Hey! I've got the best seat for this performance!"
I finding piano playing is quite a mental struggle ( as well as technical ) that is very similar to learning mediation and martial arts.
I'm wondering though do you ever feel so into a piece of music because it is so enjoyable for you that you have to break the concentration because the emotional feeling it gives you. I sometimes find a piece so enjoyable to play and hear it distracts me and I make I mistake because of its beauty .