Why is Ubuntu's clipboard so hit and miss?
Solution 1:
You may want to review the Ubuntu Wiki's article on clipboard persistence, as it discusses the problem and how to get around it using clipboard managers.
This issue also happens to be a well-known, 7-year-old bug on Launchpad. If you have a Launchpad account and feel like it, you can go there and mark yourself as affected, though I wouldn't expect it to get fixed anytime soon. This comment on the bug explains why it exists from a technical standpoint.