Masks missing across Lumetri and Gaussian Blur. Renderer locked to Metal GPU. Issue persists after preference reset and occurs in new projects.

Hi ​@tracycles 
Welcome to the Premiere community. The way masks work has changed a little. The way it functions now is by design.
Inside the effects control panel, you will see a new Unassigned Masks scroll-down tab.  Any mask that is not assigned to an effect or function, such as opacity, will now be found there. What you will want to do is make a mask using the Mask tools in the Tools panel. Once you have created the mask, it will appear in the unassigned Masks tab. You can now add your effect, Lumetri or Gaussian Blur, and then take that unassigned mask and drag it into the effect Lumetri or Gaussian Blur. I will try to upload an example video of the process.
Here to help.
Ian