Any object type CAN have a Description property, but it is optional. Similarly, there is nothing wrong with the fact that your code asks info on vendor object types, what was wrong was that when it got the unknown object response, it stopped any further requests. It should, if anything, report that error in the results, and carry on. If you are only asking for objects returned by an object_list request, you SHOULDN'T ever see that error, but BACNet manufacturers are notorious for not checking everything out, and bugs like this exist everywhere. The error just needs to be handled gracefully.
In the case of those bugged devices, those points are AV's, but are not being reported as such properly in the object_list. I'm in communication with the manufacturer about getting the firmware fixed.