I guess you were given a mix of Z and D fares because one flight didnīt have any availability in booking class Z. Generally speaking, if youīre not looking for a last minute ticket and are flexible +/- 1-2days, you should be able to find a flight with Z availability. Other than that some fares have other restrictions like Saturday night rule, advance purchase requirement, minimum stay / maximum stay, seasonality etc., however I canīt see any such restrictions for the $3300++ fare I quoted.

Btw, the fare basis is ZRC0. Iīm using Expertflyer, and you can see the info yourself if you sign up for a free 5 day trial, or if you provide the dates I can have a quick look whether booking class Z is available.