I've just updated to Xcode 9.4.1 and experienced an issue with the clipboard being cleared randomly.
So I select a text, hit CMD+C
, then go to the position I want to paste it, hit CMD+V
, but either
Update
Still remains with Xcode 10.1
Update 2
Still remains with Xcode 11
Turning off the 'Automatically Sync Pasteboard' option in the Edit menu of the Simulator works for me. I read it in Apple forums