sudo Defaults secure_path list addition

Issue

We tried to extend the secure_path default with an include file in /etc/sudoers.d, but it seems to not work. Specifically we have added /etc/sudoers.d/path with the following content:

Defaults    secure_path += /usr/local/bin

In /etc/sudoers, the secure_path is set to:

Defaults    secure_path = /sbin:/bin:/usr/sbin:/usr/bin

We expected to get /usr/sbin/local:/sbin:/bin:/usr/sbin:/usr/bin, but the result is /usr/sbin/local. Why is that the case?