OnTextEntryCompletion() works with mouse-paste but not Ctrl-V - why?

Something I discovered a while back while working with text entry boxes - pasting text into a text entry box using keyboard Ctrl-V doesn’t work. It gives an error saying there is no text on the clipboard (even though there is and will paste elsewhere). OnTextEntryCompletion() only accepts a paste input via right-click-paste using a mouse.

I have tried to dig in and figure out why with no success. What is controlling this unexpected/atypical behavior? Can it be changed or is it intentional to avoid some key command issue somewhere?