The prompt with warning comes up before the request is made to delete. A ‘toast’ will popup in light green confirming the server response.
This may be a “is it plugged in” type question, but is it still in the list for previous account? Is there any possibility that it has collaborators setup on it which would then make it persist, if they have sufficient privileges, else where? You may be able to see such information via the CLI tool.
Querying the direct API rather than the third party TTN Mapper, the gateway is still in the database - it can take a while or TTN Mapper to expire a gateway that’s been removed.
The simplest resolution is to use a different EUI. Is there any reason not to do this?