Not sure about the “wrong difficulty when selecting quickplay” part, but there is a different bug with the Mission Board, that causes you to start quickplay, when you had a specific mission selected.
Apparently, your map selection is reset when ever the mission board is rearranged upon any of the missions disappearing. This “selection reset” is not reflected by the interface though.
This results in a quickplay mission being started although the interface still showed that a specific mission was selected.
Click on a mission on the board. Any specific mission is fine, but do not select the mission with the shortest time remaining (your selected mission has to stay available).
Wait until the mission with shortest time remaining disappears from the board, causing the remaining missions to be rearranged. Your mission is still available and according to the interface, you still have your mission selected.
Click “play”. You think that you should load into your selected mission, but instead, you will start Quickplay.