It is, yes - this still looks like a discovery problem, but the IP addresses look consistent now so it's not that.
Can you share the precise command you are using for this test?
Could you run curl -vv http://10.203.116.$N:9300/ from within this container (choose $N for one of the other master nodes) and share the full output too?
No, I think that's to be expected while the cluster is still forming.