Mail loses keyboard shortcut for "Mark unread" after using it on conversation

When I have "Organize by Conversation" set in Mail and I click on the top of a conversation group which brings up all the messages in that conversation, if I have a particular message in view and I try to hit the keyboard shortcut for "Message/Mark/As Unread", Mail beeps with an error, which is fine since I don't have a message selected. But then when I do subsequently pick a message and try to hit the keyboard shortcut for Mark As Unread again, it doesn't work like it's been deleted or changed. The only way to "fix" this is to select with the mouse the menu "Message/Mark" and then I can hit the shortcut again and it seems to resussicate it.

This is regardless of which keyboard shortcut I have set in System Preferences.

Anyone else see this bug or have a remedy?