The problem reoccured with 15.3. I was puzzled because it only happened with stereo Homepods that are default output for my TV 4K. A single Homepod in another room worked well.
Eventually I found that I needed to:
a) change the ATV default output to the TV
b) go to the stereo Homepod settings in the Home app on my iPhone/iPad
c) go to Music & Podcasts/Primary User
d) select myself as the Primary User (not Homepod Account) but...
e) enter my AppleID & password for the "Homepod Account" (leave myself as Primary User)
f) receive a login error message
g) enter them again - this time they were accepted (maybe because there are two Homepods involved?)
h) test it with a Hey Siri request
i) change the ATV default audio back to the stereo Homepods
j) test Hey Siri again - it worked for me!