Macbook 16" won't extend desktop with Display Port Chaining
macOS does not support MST (Multi-Stream Transport) daisy chaining, so you cannot get such a setup working with extended desktop at all. You can get mirrored displays working (i.e. both monitors display the same picture), but this is obviously not what you want. 

You can get extended desktop working in a MST daisy-chained setup as you have by running Windows or Linux on the MacBook Pro.

As you have figured out yourself, you could connect both displays with extended desktop by using two separate cables that both plugin to the MacBook Pro.

If you want to use a single cable to connect multiple monitors in extended desktop on macOS, you'll need to use either Thunderbolt monitors (those support daisy chaining), or a Thunderbolt dock such as the OWC Thunderbolt 3 Dock or similar.