Yes, self-signed certificates are rejected, because there's no way for the server to confirm the identity of the other end of the tunnel; thus, there's virtually no added privacy benefit (at least no privacy guarantee, and some regulations/expectations mandate SSL/TLS certificate authority signing in some cases).
Consider it like this:
1. Hologram Cloud sees you are using HTTPS, so privacy appears to be a goal
2. Hologram Cloud sees that there's no way to prove the identity of the message recipient, so it doesn't send the payload, due to #1
The two solutions are either to use HTTP or to obtain a certificate-authority--signed certificate. You can obtain free signed certificates from LetsEncrypt including customizing their expiration dates.