I have two Fedora 29 systems with xfce4-panel 4.13.3, on either when I hover over notification are icon, the tooltip is not shown. It worked fine on Fedora 27, with xfce4-panel 4.12.1. However, I cannot easily downgrade to check it out.
So, now I have tested it with Fedora 28 and xfce4-panel 4.12.2. Also works as expected.
*** This bug has been marked as a duplicate of bug 14967 ***