I notice that the “Show all masters” menu item does nothing when a glyph is selected (e.g. by clicking before it and shift-clicking after it) in Text mode. It’s only when there is no text selected that it produces the masters for the glyph right of the cursor location.
I think it would be ideal if I could select multiple glyphs and have the command produce the string in all masters. But for now, it seems to me like if there is only one glyph selected it should go ahead and work as you’d expect. (If for some reason it’d be better if it didn’t work, the menu item should be grayed out if it will not do anything in that circumstance.)