Yep - I have OGs paired in several rooms, and used to be able to reliably airplay a local playlist to several rooms at once from my iPhone. Since iOS 18 I see the same behavior you do. Of interest is that I can still airplay to multiple targets from my Mac (still running Sonoma) without a random HomePod emitting static/fart noises.Anyone else having significant new multi room or stereo pair issues with iOS18?
Honestly I dread major OS updates these days. Every year, somehow, Apple breaks stuff that used to work perfectly well. I mean, why even mess with that code? It makes no sense to me.
Anyway, you are not alone. Hopefully 18.1 will fix it.