Is it currently possible to create custom asset bundles?



Unfortunately documentation and/or examples about this topic is close to nil. I’ve tried to follow the old and seemingly outdated manual, but, after editing bundles in .xpkg and adding some tags to my prefabs, all I get is a build error like this: "Can not find 'correct/relative/path/to/MyPrefab' for 'MyCustomBundle'".

So… it seems it’s picking up tags correctly, but then fails to find the asset itself, despite knowing its valid URL.
The only workaround is to include assets as root, but imho that defeats the whole purpose of bundles.

Am I daft and missing something, or is it broken in the latest ?

Thanks beforehand for any pointers.