Reproduction steps:
1) Open attached project "ABRefBug"
2) Build asset bundles using the menu "Assets/AssetBundles/Build AssetBundles"
3) Open "Main" scene
4) Select "Loader" object
5) In the inspector check "LoadWithExtention" only
6) Enter play mode
--- Result is logged in the console
--- You should see that one of the assets with the same name but different types failed to load

SomeAssetDefinition someAssetDefinition = bundle.LoadAsset<SomeAssetDefinition>(assetName);
SomeScriptableObject someScriptableObject = bundle.LoadAsset<SomeScriptableObject>(assetName);