I'm having the exact same issue. I can access the two HomePod minis in my home office only from my iPad and my Mac(s). I cannot access them from my iPhone 14 Pro or my iPhone 15. Both iPhones are on iOS v18.3.
I could play music to them from the MacBook Pro (macOS Sequoia 15.3) and from the iPad Pro (running iPadOS v18.3).
I had both the minis paired for a Stereo Pair. I deleted the pair and restarted both minis. After that, the iPad and the MacBook Pro could still see them individually, and could show them in the list of AirPlay options, too. But neither iPhone could connect to them. They show up in Home on both iPhones, but when I click on their icons from the iPhone, it reports "Controls not available" and clicking the gear icon shows all of the options greyed out. When I do the same from iPad or the MacBook, I can connect to and control both minis.
When I bring the iPhone close to either mini, it brings up the proximity option, but it still shows all controls greyed out and report "Controls not available."
It certainly seems to be an iPhone and/or iOS issue.