The problem is now fixed and was being caused by 'New AI Behaviour'. I did some trial and error and the problem was specifically caused by 'Cache Generated Payloads' being enabled. All other new AI stuff added with CSP seems to work perfectly fine so I've reactivated it.
Awesome thanks! This was the reason why my sidelines wouldn't bake into the fast_lane without pressing shift each time -> I couldn't share my fast_lane for my track if they didn't have the AI Line Helper.
Now it works flawlessly, much appreciated.
Also big thanks to the dev. I have no clue how I would've finished my track without this tool. It has improved sooo many things and even some modded maps I downloaded which had broken AI!