From what I know, in order for a BJT to operate:
The emitter-base should be forward biased (current mirror's Q1 Emitter-Base is forward biased, so its okay) and the collector-base region should be reverse biased (current mirror's Q1 collector-base is not reverse biased but it is short circuited so its not okay).
I can't also just rely on the equations Ic=beta*Ib because for this equation to be true, the B-E should be forward bias while C-B should be reverse bias. And in the case of current mirror, C-B is not reverse biased.
It will be more clear if you can explain or describe in atomic level whats happening on Q1. I understand holes, donors, etc.