Using umbrella-0.20.0 did not kick off a stable docs build on RTD. From their documentation, a stable build is only done when a semver-valid tag is pushed:

You should push a tag for each version of your project. These tags should be numbered in a way that is consistent with semantic versioning. This will map to your stable branch by default.

I tried to force it to rebuild but it stuck with the last valid tag at 4d66524.