Is it normal i feel left out of nude situations with my stepdaughter

I would like to preface that I've been with my wife for almost 18 years and have helped raise hers (our) daughter and son since they were 12 and 10, respectively.

On a recent trip overseas, our family, wife, stepdaughter and her fiancé went to a German spa with sauna. My wife was unable to attend due to an event, she told us to go ahead and enjoy ourselves. When we went to the sauna area, my stepdaughter seemed to become uneasy, saying it was awkward for her to cross boundaries because of family issues and nudity, but it was different with strangers. I was disappointed with her decision, seeing as I thought she was an open minded person. Her and her fiancé left and told me to enjoy and we'll meet up in a couple hours.
She is open, just not in front of me, though she's seen me full monty on a couple occasions accidentally, including the sauna that day when she and the fiancé were looking for me so we could go to dinner.
After dinner, I decided to leave early and let them both enjoy the sauna together.

I just don't understand the logic of allowing strangers to see her naked, but not her stepfather, who she said was more of a father than her real one. I used to watch her walk around in her underwear without a problem until her mother told her to cover up because of puberty.
I hear stories from her girlfriends about how wild she was at this party or what about when she and 3 friends walked naked through a public park.
Feeling left out