Same issue here, although I have noticed the behaviour is different depending on how I respond to the meeting request:
1. If I respond directly in the Mail app, the response will go out to the organizer. However, I will not see the response in the sent folder. Mail deletes the original invitation from the inbox.
2. If I respond from the Calendar app's "inbox", the response will not reach the organizer and instead end up in the Mail draft folder (similar to everyone else in this thread). It will have the "no sender" issue, but if I open the response I can actually send it normally and it will show up in the sent folder. The original invitation is still left in my Mail inbox.
3. If I respond by opening the actual calendar entry (the grey one with dotted lines), the response will go out to the organizer correctly, and I will have a copy of the response in my sent folder. The original invitation is still in my Mail inbox.
All three methods will generate the appropriate entry in my calendar. Number 3 is what I would prefer... but obviously I would expect all different ways of responding to work in the same way. Also, I think many users would like to change whether the original invitation is deleted or not.
I'm new here so my question would be whether this thread can be sent as feedback to Apple?