Hard to say if this is a feature or a bug but it has been reported a couple times. If you have copied a takeoff to your clipboard then open a new tab your clipboard will be empty. If you have two tabs open you can copy takeoff from Tab A and Tab B and then past The same data on each tab. I cant copy on tab A and have the clipboard paste that onto tab B. Tab B has it's own clipboard info.
This might explain it better:
https://youtu.be/MbTXjPhCX1k
Thanks for reporting. The browser tabs do keep track of their own "clipboard" currently. We'll dig into how to fix this. It's a little tricky since it's not actually copying to the true OS clipboard for the section data (otherwise it would be a huge JSON data object when you paste into another app such as notepad, which might not be all bad). I'm sure there is some work-around to solve this, though. It gets a little more tricky when it's copying data between projects, etc. but should be possible still.
I believe this was fixed a few releases ago, but you should now be able to copy a takeoff, open a new tab, and paste the copied takeoff.