I'm gonna write a list of everything that is required for the scar fight, tell me if anything I mention is something you haven't done.
1. Get the attack on bareshade event and choose between discouraging or encouraged him. (This is something you already said but I'm just mentioning everything in order to see if you missed something or the game is bugged. Also if discouraged the idea do his defenses for bareshade event.)
2. Explore the bandit camp at night and look for jester, go with him and do almost all of his interactions, also look for an event of tank trying to fuck another bandit, After that explore the track and encounter scar and gunnar talking about northcrest.
3. After that interact with jester again and talk about it. This should then unlock his "I need to talk to scar" interaction.
4. Once done he should tell you about it being a good idea to sneak in during the day for a boss fight.