copy paste loses contents after a few seconds

My copy/paste is broken, even within the same app.

I can ⌘-C copy text in TextEdit, then ⌘-V and it shows the menu item being used ([edit] menu blinks) but nothing gets pasted.

Then I try again - reselect the text, ⌘-C & ⌘-V and it works, but if I keep pasting, it stops pasting after ~3-4 tries.

Now I must re-copy and pste almost everything 2-3 times to get it to finally work, and do it quickly by using the app-switcher/Tab-switching/window-switching keyboard shortcuts, as if I take too long the clipboard loses it's contents! The first time alway fails, even if I switch and paste quickly, I always have to do it twice at minimum.

I have tried Force-Quitting `pboard`, and restarting the computer - issue does not resolve.

What else can I try?

FYI this happened recently after upgrading to Mohave, 10.14.2, from High Sierra Previously. Not sure if that's the root cause or if it's a red herring.

MacBook Pro (13-inch, 2017, Four Thunderbolt 3 Ports)