Fitness+ on AppleTV recognizes iPhone but only allows "Guest" access

Since updating to iOS 18, our AppleTV Fitness+ app does not recognize my wife's iPhone. All we see is my phone (account holder, which works fine) and a generic "iPhone" option for her phone. When we select this "iPhone" it asks permission to open Fitness+ on her phone as usual, but then it only allows her to proceed as a guest and not in her account.