That kind of split is pretty normal when the same post is distributed differently on each platform. A Facebook share can travel through older audience networks and reshares, while Instagram may be giving more weight to watch time, replays, saves, and profile actions from its own viewers. The raw follower jump does not necessarily mean the post failed on Instagram.
I would compare the two insights screens for that exact post: average watch time, completion rate, shares, saves, profile visits, and where the views came from. That will tell you whether Instagram is reaching fewer people or whether people are seeing it but choosing not to follow.
For the next few posts, keep the core idea but adapt the opening and caption to each platform instead of cross-posting unchanged. On Instagram, make the first frame explain why the right viewer should stop, then use Stories to follow up with the people who watched or replied. Three weeks is still early, so I would look for repeatable signals rather than expect the follower counts to match.