In my experience, T's and p-docs handle sharing information in different ways. I used to see ex-T, ex-marriage counselor, and ex-p-doc in the same practice. I did sign a waiver saying they could all communicate with each other. With ex-T, she would ask my permission before she shared anything with either of them. Meanwhile, ex-p-doc shared an incident with them that I would have preferred to tell them about myself (well, I had told ex-T myself). With ex-MC, I didn't think he was really sharing any info. Then later, not long before he terminated, he said that he often shared things with ex-T. Which kind of bothered me, because neither he nor ex-T had mentioned it. So I had no idea what he'd been saying.

With current T, who is in solo practice, I was pretty restrictive in who he could talk to. I never gave him permission to talk to ex-T. I gave him very restricted permission to talk to ex-MC (when I was still seeing him) in that ex-MC could share info with him about me, but T couldn't share info about me with him (there were options on the form). And I eventually let T communicate with current p-doc, but he informed me that he'd be calling her, and he told me afterward what they talked about. And p-doc also mentioned what he said to her. I think much of it for me (now) is about being transparent in how often they're talking, what sorts of things they would share, under what circumstances, etc.

I would let your T know how much this bothered you and request in the future that she tell you if she plans to tell p-doc about something (and you can request the same of p-doc). And at least give you the chance to tell them first. If you don't feel comfortable with her answer, you can always rescind your permission to have them talk (I think you're in the US, right?) If that's an issue because they're in same practice (I still had to give explicit permission for mine), then consider seeing a p-doc outside the practice. I think whether you can have faith in her depends on how she responds to all of that.