Lightsail - Can't delete static IP address, reverse DNS record attached?

I've deleted everything in my Lightsail account except a static IP address which was associated with an mail server instance I ran on the platform. When I try to delete this I get this error:

ReleaseStaticIp[eu-west-2]

The specified Static IP address cannot be released from your account. A reverse DNS record may be associated with the Static IP address. To unlock the address, contact AWS Support.

InvalidAddress.Locked

I have deleted the DNS A record that pointed to the static IP (confirmed with dig) but still I get the error.

Is there a long delay between deleting the record and Lightsail picking up the change? It's been several hours since I made the change, Google DNS saw it immediately.

Annoyingly with the free tier I can't get any techincal support from AWS.