Current processing of page start will load course slug as session cache named 'current_course_slug', it's the serious conflict with page study.
It means if the page study has been opened, then users start any other course and into page start again, system will be rewrite session cache, study system can not get its course's slug.
So, fix it by below:
Key 'current_course_slug' have to just be used as study system session key,
Page start have another key as session cache, named 'start_course_slug' anymore.
Current processing of page start will load course slug as session cache named 'current_course_slug', it's the serious conflict with page study.
It means if the page study has been opened, then users start any other course and into page start again, system will be rewrite session cache, study system can not get its course's slug.
So, fix it by below: