So, I’m not sure what exatly is going on with the experimental version.
Have a look at the distribution releases, on bookworm (which has the stable release), you can see the version is 0.4.8.12, which just got released.
The nightly version is 0.4.9.0-alpha-dev, which is expected.
However, if you look closely, the experimental version was last modified one week from now (probably due to some automated process), but the version is 0.4.7.6-rc-1, which dates back to April 2022.
The reason it still downloaded version 0.4.8.11 even if I added the experimental repository is because it fetches the latest version, which happens to be the stable.
I wonder what happened. Apparently the experimental versions are no longer used, or at least not made publicly available before it becomes the stable release.