Yeh, we have. I have did and some alters aren't either impressed or engaged with the whole therapy thing. So sometimes when they have been triggered out they just get up and leave. In fact that is one of the main reasons our T suggested we record our sessions... because when we switched out we often had no idea what had happened or how therapy ended. A couple times an later left after only 20-30 minutes, other times it's been closer to the 45-50 minute mark. Sometimes we have switched back quickly and returned to the therapy room, other times the alter has left completely. T never chases us down... we either go back or we don't.
It doesn't happen so much anymore.

Btw... fwiw, I would call leaving 2 minutes early "leaving on time". But I respect that it felt rebellious for you.