I just tested an application that allows to see the pasteboard (pasteboard Viewer).
And I realize that at the moment of a selection there is this that is activated:
org.nspasteboard.transienttype
org.nspasteboard.AutoGeneratedType
does it tell you something ?
I should have done what you said before.
Killing all the applications.
once a big part of closed, BTT reacts correctly.
It's popclip who does it 