Hello,
I tried to replicate the issue but was unable to do so i.e. I was able to copy the value from a combobox cell and paste in multiple cells.
I would request to you kindly share a sample replicating the issue so that I can assist you further.. Since you are upgrading from a very old version, so there have been many changes like new features, bug fixes etc. in the control over the period the time, Moreover, you have override the ProcessCmdKey so it might be causing the same.
Regards,