That can be disabled or changed where the threshold is, so this is normal.
No saturation is different to how bright a globe is. You can have pastel colours that are very washed out red for example but the globe is still bright. Saturation is the ratio of the white LEDs VS the raw colour/s.
You would need to look at the openHAB event log to see what command is being sent to the binding. The developer sidebar is one way to do this and it allows for filtering so you only see what you wish. I am guessing that google home is sending a brightness command to the event bus VS your sending a saturation change. You can link multiple controls back to the same channel so if you wish to have a separate brightness slider and color controls you can do this.
Do a search for OH3 widgets as there are a number of custom widgets people have created, one example is this one.
OH3 Color/White Bulb Widget - Add-ons / UIs - openHAB Community