The Paste function is sent directly to the Windows, so it will do use whichever handler it is registered. You can set from TeraCopy to "Use TeraCopy as a default copy handler" and it will work also for pasting from OC.
It is not a bug, but it just not implemented to explicitly take over the paste command also. It is only inconsistent if TeraCopy is not a default handler also in Explorer. Please let me know if it works as expeceted if tick that checkbox in TeraCopy.
It is kind of a Pro feature. I am asking myself for everything: Would my father need this, or my wife, or brother in law, and a few other close people who are also just casual computer users, and if they would not use that feature or even know what that is, then it is probably an advanced feature, and they would still have a fully functional program even without it.