The package panda3d1.9 installed from

deb http://archive.panda3d.org/ubuntu xenial main

sets the permission of the man pages to 600. This prevents normal users from accessing the pages and also prevents mandb from updating the manual page index caches.

Simple fix (while waiting for the package to be changed):

for i in $(dpkg -L panda3d1.9 | grep "/usr/share/man/man1/")
do
  sudo chmod +r $i
done