Proficiency to damage roll was my mistake. I realized after posting it that I was incorrect about that.
The reason why I thought the str modifier wouldn't be added to the damage though is because it's not with the rogues bonus action. Theirs being the second attack made with their second weapon.
I am a newer DM just getting I to DnD with the latest edition here. One of the players in my group has taken the role of fighter and using the human varient he started with the polearm mastery feat which allows him to make an opportunity attack on creatures who enter his range, and add an...