As mentioned before selecting the select tool again cancels the bug. BUT only if you select the select tool in the buttons at the top or by right clicking in the project area. Selecting the tool with key commands does not fix it.
Not knowing anything about the code in Cubase and Nuendo. This still leads me to belive that the bug (if it is a bug) has to do with the tool/modifiers and how that interacts with the interface. And that the solo/mute logic is probably fine.