I need to raise a case and check up on this but I believe this is what's causing it. In a similar fashion to any slot, if you leave before the round is over it gets kind of "stuck" and you can play it when you open the slot again.
In this case it'll just leave the "free spin" remark there as the round is not over when it actually is. But I'll check this with people smarter than me tomorrow and let you guys know.
This is the Reel Investigator promotion, correct?