There seems to be a minor but confusing change to creating a Homekit Scene and requesting Siri to activate that scene.
I just created a new scene for my garage and it tested successfully.
I then asked (Homepod) Siri to activate that scene but she responded that it could not be found.
On checking the Home app I found that the scene was listed in the Garage room but not the Home screen. I edited its setting to add it to the Home screen and Siri now recognises it.
So it seems that only scenes that appear on the Home screen are available to Siri. The settings of newly created scenes need to edited to add them to the Home screen.