What you describe is definitely related - in fact that’s how I discovered the problem. I’m usually not using write-automation as I’m more often automating manually and only rarely do I “record” the automation (the latter needs write-automation to be enabled, the former is not). I only recently started using “linked panners”, and when I happened to record a panning automation with “write automation” enabled, I noticed that cubase actually created two panning automations - one for the “main” track pan and one for Send #2 panning (send #2 happened to point to an effect track).
Seems to me this hints the implementation of this “feature” is somewhat strange or buggy - consider the following two scenarios:
(1) What if I happen to work manually without having write-automation enabled?
(2) Even if I would always have write-automation enabled, what if after I already added a pan automation I am adding a new SEND? Will Cubase automatically duplicate the pan automation I already have?
The answer for both of these questions is “no” (I checked it)