OP i would be very very pissed off.
She lied by omission.
She also took your choice away.
I would not trust her again.
I have had a similar -ish situation, when my child was having an evening with pal, there was a group of them.
The Mum had to visit her mother unexpectedly, she immediately sent out a group text, that her husband would be in charge.
That is what an on the ball parent does....and we all knew her husband well.
But she was being respectful.
I find it very dishonest.
Yes, you probably have learnt a lesson to specifically ask now in future if said parent will be in situ ,but it is really bloody annoying that you have to.
Any normal, decent parent, would not do this.
Trust in that womens judgement would be gone for me.