I'm having odd behaviour with my Homepod. It feels like a bug. It has internet (plays music fine) and is definitely on the same wifi network as my iPhone and my other mac computer (I've verified this in my router admin area, and it is not on a guest network or anything that might have network isolation). It shows up fine in the 'home' app of my iPhone. However it randomly disappears as an airplay option (from any other device), and if I try and a do something that requires my iPhone like add a reminder I get the 'Before I can help with Personal Requests, the iOS device associated with this HomePod needs to be on the same Wi-Fi network.' message.
Note I'm on the latest firmware and iOS as of today (12.x)