Ex Unblocked Me On Instagram But Didn't Follow Me

Less common, but possible: they want to monitor what you share without signaling any interest or accountability.

If they are genuinely interested in getting back together or being friends, they will take the next step. They will follow you. They will message you. Unblocking is the bare minimum. Do not reward the bare minimum with your attention. ex unblocked me on instagram but didn't follow me