"I will not attend" never works

I'm using TB 17 beta and Lightning 1.9 b2.  I'm connected to a corporate 
Exchange Calendar using DavMail gateway 4.1 (latest version).   Most 
things work in this setup such as adding/removing/modifying a calendar 
item and all is well in that regard.

When responding to an event that was initiated from an Exchange user, I 
get the invitation, and if I respond with "I will attend", Lightning 
does what seems needed and all is well, but if I try to respond with "I 
will not attend", nothing happens.   The UI appears to accept my choice, 
and I click OK, but no notification is sent (or offered to be sent) no 
message appears in the TB error console, nor is there any change to the 
event.  If I re-open the invitation, it is still waiting for me to make 
a choice - which is incidentally good for the other folk because my only 
useful choice is "I will attend"  ;-)

So is it a bug in progress or something new?