It is an orange D. The solo function stopped working with this sudden denotation I have never seen before. Also I have the strange experience of having channels solo with others that are not grouped. (I saw that was bug in 14 and hope it was fixed.)
The ‘D’ shows when you Option Click the Solo button.
It means its ‘Solo Defeated’ - ie it is always audible.
Hover the mouse pointer over the D in order to get a tool tip on screen that shows how to get rid of it.