It should not be expected for standard user to access services in ipv6. Rapid.Space and generaly SlapOS infrastructures contain an IPv4 frontend used to access an IPv6 backend transparently.
Most software release request an ipv4 frontend automatically for users to access the service. If the Theia you are using have been provided on Rapid.Space, you are accessing an IPv6 service through an ipv4 frontend.
This tutorial explains how to request an IPv4 frontend automatically in the context of html5as-base.
Here is the commit introducing this functionality to html5as Software release: commit diff