Minor bug:
Mute envelope can have linear shape segments when automation items are used. It should have only square shapes.
Another old bug related to mute envelopes:
After using a mute envelope, the "Set point value..." dialog has only "Square" shape in the drop-down menu. This happens in all envelope types in all projects. You have to restart Reaper to get all shapes back to that drop-down menu.
jnif