DownloadDependencies appears to be broken. I confirmed what this user said in a post I made about a similar issue you’re describing.
I finished making an addressable test environment to perform regression tests on addressable packages updates and I verified that DownloadDependencies does not load the bundle into memory, it only downloads it, so by the time LoadSceneAsync is invoked the “unable to load scene/bundle error” occurs. The error started to occur after I upgraded from 1.19.19 → 1.21.17.
I haven’t thought about using LoadAssetAsync to manually load the asset but I can see this being an issue if you have more than one dependent bundle that needs to be loaded. I’ll try it out anyway for testing sakes.
Here’s another post with a user having a similar issue after updating to 1.21.17.