All other ports paste primary selection on button release, not on button press. GTK port does it on button press to match GTK's behavior, but this doesn't work well in combination with Epiphany mouse gestures. The path of least resistance is to just change it to match all other ports. See https://gitlab.gnome.org/GNOME/epiphany/-/issues/1814.
Pull request: https://github.com/WebKit/WebKit/pull/6048