They may have changed the format for node names recently:
https://community.fly.io/t/env-sh-eex-and-what-to-do-with-it/14349
That post shows…
<APP_NAME>-<IMAGE_REF>@<IP>
as the format for RELEASE_NODE, whereas the name in your log snippet follows the older <APP_NAME>@<IP> structure.
https://community.fly.io/t/elixir-phoenix-libcluster-unable-to-connect-anymore/5845
A reply by chrismccord (of Fly) in the July 2023 thread said that it was to ensure “unique[ness] across deployments”; maybe wires got crossed there?