I built Xfce-panel 4.14.1 for LinuxMint 19.3 'Tricia' (which has Xfce 4.14 by default) GTK 3.0-3.22.30 , as I had the same issue. But it still did not work out of the box. On checking bug #16106, I enabled compositing, brought the Enter and Leave opacity to 100, disabled Compositing, and restarted the panel and only then did this fix work for me.
I built Xfce-panel 4.14.1 for LinuxMint 19.3 'Tricia' (which has Xfce 4.14 by default) GTK 3.0-3.22.30 , as I had the same issue. But it still did not work out of the box. On checking bug #16106, I enabled compositing, brought the Enter and Leave opacity to 100, disabled Compositing, and restarted the panel and only then did this fix work for me.