Try installing GSAM battery monitor if you haven't tried it yet.
Then you can compare your stats and data against other users Z Ultra's performance and judge, whether your unit is unique or not.
If you look into which apps are preventing your stamina mode (if u are using it, I do not know) activating, thus using system resources all the time despite the fact that your screen is off and assumed idling, you'll know what to do and how to take things forward from there.
Apps that need to send and receive data tend to be the culprit. Also some task killers tend to use unnecessary resources too...