Daughter [3f] calls another man "daddy". I'm [27m] afraid fiancee' s [25f] relationship with the man is replacing mine?

Fiancee and daughter go around the city a lot to do different things when I'm at work. Yesterday I had the day off, so I went with them to a painting class they go to. There were other parents with their kids there, and we sat next to this dad and his son. Right when my daughter saw him she jumped up and down, waved, and said "hi daddy". My fiancee didn't correct her or anything, just exchanged a hug with the boy and dad.
It was like almost the whole time we were there they barely said or paid any attention to me. My daughter seemed to be much more interested in them, what they were doing, and just what the man was saying and how he was interacting with her. My fiancee was practically giggling at everything he was saying and doing, and afterwards when we left daughter said "bye bye daddy".
When we were in the car fiancee said that ever since they attended that class she's called him that, and despite the many corrections she hasn't stopped, so she just ignored it. I told her how i felt a little left out, to which she just laughed and said because it was my first time there. But the man didn't acknowledge me or anything, despite me being right there. I don't know i just got a strange vibe from him, and I just feel like my daughter is seeing another man as daddy because of all the time they spend with him. They go on play dates together, lunches, and places like the zoo and museums. My Daughter and his son are best friends, but I feel like all that time has replaced my relationship with my daughter? I don't know, is this something I should bring up with her?
tl;dr: daughter and fiancee spend a lot of time with this man and his son. I don't mind it but I feel as if they're relationship is replacing the relationship I have with them. Is this something o being up?


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