Note: I think it is important to differentiate opinion from fact. The following is my research-based opinion:
My conclusion now is that it is NOT the iOS, but that recent changes at Google have had some unexpected results. Google finally retired Google Play Music in favor of YouTube Music which is a paid service. Early on, I uploaded a part of my then music library to Google Play, which was then transferred to YouTubeMusic when it became time to sunset Google Play. I quickly discovered my that I can stream songs that I own, but I cannot stream anything else without a monthly fee. It was at the same time that Chromecast stopped streaming from iOS. Here is the thing: iOS knows the devices are in my network. But they are unreachable. If this is a bug and not a feature as they say, then it is a Google problem. More specifically, I think is is a DRM issue. See the shot below. My RadioTunes app on iOS wants to stream. But the INDIVIDUAL devices are not visible. 