A possible solution in my case…
I kept my Homepods awake all night by putting an album on repeat. Last night all automations worked when they failed on previous nights (back to the OS16 update). So it is possible, in my case, that Homepod hubs going to sleep is causing Homekit problems.
Update: I have created an automation to (I hope) keep other automations working! The automation triggers late evening and plays a selected song through the stereo Homepods at near zero volume and with repeat enabled. Not an elegant solution but it is easy to test until a permanent fix is out there.