How do I enter custom values for the population multiplier under the advanced zombie settings?

[ember brook]

When I host a world for me and my friends I am able to enter a number into all of the settings, however on solo I am only given a drop down. When I click the "custom" option on the dropdown nothing happens. Seems like maybe I don't understand something but I can't find anyone talking about this in the server nor on google.

of course I've had this issue for weeks and as soon as I fucking post this I notice the little unchecked advanced box at the bottom

I feel like this kind of defeats the point of "advanced zombie settings" though