Over the past 20 years (or more) in data centers, I have encountered many scenarios where a setting is selected or deselected, and you don’t get the result you expect.
I’ve had this issue with Spanning Tree, speed, routing protocols, Wi-Fi channel, and countless other settings that drove me nuts. Now, when I see something I don't quite understand, I capture some packets (when possible), so I can get a clue as to what's going on under the hood.
In this specific case, the client disabled the DHCP server, but I showed him that it was still active and assigning addresses.
Similar issues are likely to happen all the time. Instructions, manuals, and people may all say to do one thing, yet the outcome is not what you expect. Moral of the story: When in doubt, test, test, test.