Having the same problem with my BOSE Soundtouch speaker.
Like everyone else, I believe the issue is with iOS, in my case the issue occurred in iOS 13 and still present in iOS 15.
I have 3 BOSE speakers. One uses Airplay 1 and works fine, the other 2 uses Airplay 2 and only 1 of them works consistently.
I have found that when I reset the speaker with the Airplay issue (ST 300 soundbar) it works but only for a short while, maybe a day... then it drops off again and starts asking for an Airplay password, nothing I input seems to work. I've googled high and low and nothing suggested seems to fix the problem. I did read something that maybe it's a router problem, and I might have too many devices connected which if I'm honest that might be a possibility. But I'm not sure how I would go about solving that.
It's a right PAIN resetting the speaker every time, as each time I have to also recalibrate my surround sound speakers and sub! In the end I've just given up and resorted to using the soundtouch app and/ or bluetooth.. not seemless like airplay but at least it works. What a shame.