Since the update V4.0.86.2605190956 of Razer Synapse on Windows, the button 9 of the Razer Naga V2 Pro mouse cannot be modified at all.
Here’s what button 8 looks like, what is expected:

And here is button 9. Notice that the turbo function simply isn’t present:

Since turbo was activated prior to this update, it still works. However, this update changed something in the behavior which make the function behave extremely herratically. To fix it I had to tone down the turbo on the button 8 for scrolling up. However, due to this bug I cannot do the same for button 9.
A partial fix comes in the way of closing Razer Synapse completely. Even though both of these function uses Razer Synapse, closing it seems to make it behave non-herratically as it did before. However, I change between two profile often so having to re-open Razer Synapse, change profile and close it again is not convenient at all.