It sounds like you may be distributing outside the App Store. Almost all my iOS work is distributed using Enterprise profiles, and using tools other than Xojo (in part because the projects pre-date Xojo support for iOS, and require features not easily done in Xojo yet.).
But back in 2019, I did experiment with a project and to get the Enterprise profile to code sign an *.ipa had to jump through a few extra hoops. You can read about some of what I did in an old thread here.
I can’t tell you if the process works the same now as 2019, but the thread may give you some hints. Review the whole thread; not just my post.