Something similar happened to me about three weeks ago. I sold a domain that I had at my registrar Namecheap. The transfer went through smoothly and whois showed the domain was now at the new registrar and it showed the buyer's registrant details. So the deal was finished and domain entirely in their new registrar.
A couple days later I was in my Namecheap account... and noticed my 'sold and transferred' domain was still there in my account! I did another whois and yes, the whois still showed the new registrar and registrant... so why was I seeing the domain still in my account, and able to access that domain's settings page, etc? I opened a chat with Namecheap, who looked at my account. They saw the domain was still in my account; and they said it was 'apparently a ghost listing'.
Whatever that means. I guess it means that some glitch allowed the domain to be transferred out but for it also to not be auto-removed from my account. I imagine that it would have stayed there until renewal time if I hadn't noticed, and maybe the 'ghost listing' would have auto-renewed in my account, even though I no longer owned the domain. Maybe that's what happened with your domain? A glitch that led to a lingering 'ghost listing'?
Namecheap's always been great for me, so no worries. This is the first glitch in my entire history with them. They manually removed the ghost listing from my account and all is once again good in the world.
Hey, somebody go register GhostListing.com