Openapi 3.0.x basepath undefined error

Update: for current state and what is left to be done, see thread.

Summary

Openapi.yaml file that shows de api rest calls stopped working properly after a few days.

Steps to reproduce

I think it happens in any project that uses openapi version 3.0.x

Example Project

It happens both in my private repository and in several public ones that I have checked to verify that it was not an isolated error.

You can see it in your own repository https://gitlab.com/gitlab-org/gitlab/-/blob/f4f2034849cc7e10836b7ca72f6ade092949ba53/doc/api/openapi/openapi.yaml

What is the current bug behavior?

Could not resolve reference ($ref): Tried to resolve a relative URL, without having a basePath.

Output of checks

This bug happens on GitLab.com

Possible workaround

As a workaround, we suggest using the local references in your OpenAPI specification file.

If they're generated automatically, please generate a single file. Example: https://gitlab.com/gitlab-org/gitlab/-/blob/master/doc/api/openapi/openapi_v2.yaml

Potential fix

A potential fix would be to resolve all the references in the backend and the frontend receives only the single specification file. This would be rendered without any issues.