I'm seeing the same thing. It works fine when driven by inputs like your constant or a noise, but yeah, it seems like the Mix parameter on its own is borked.
Hypershader Can you send me over the release note describing the fix please?
Fixed a bug with retranslating a shader node parameter when it's set back to adefaule value, which would not get reflected in the USD shader on the stage. Wed. April 27, 2022
Hypershader Can you send me over the release note describing the fix please?
Fixed a bug with retranslating a shader node parameter when it's set back to adefaule value, which would not get reflected in the USD shader on the stage. Wed. April 27, 2022