It’s hard to tell what the issue is without context on your current setup. However, it does appear that the issue you’re experiencing is similar to this one. Can you check your dockerfile settings to make sure these lines were included? Flyctl generally appends these lines [see below] to the end of the dockerfile, but it may not have been set if you customized your Dockerfile or launched your app without one.
# Appended by flyctl
ENV ECTO_IPV6 true
ENV ERL_AFLAGS "-proto_dist inet6_tcp"