The autoloader won't load files in the folder assets as already stated it is all explained here. Your asset bundles should be in a folder called resources otherwise Yii won't register them in its classmap automatically.
Your namespaces must represent your folder structure. The namespace ns\prefix\controller must be for a file in vendor\ns\prefix\src\controller so your namespace ns\prefix\assets will search for a file in ...vendor\ns\prefix\src\assets but you have to include the resource on your own via require_once since Yii won't so it because you are in the wrong folder
The easiest solution would be to create a folder resources and use the namespace ns\prefix\src\resources
Or you can - like stated in the docs - use your plugin root path as well recoveryfinder\entriesbygroup