I have no idea why you're seeing that behavior. It works as designed when I switch back and forth from fixed to automatic. Very odd.
It may be a bug, so please wait for the nest patch and download and update to see if it resolves your problem. Out of interest, do you see this issue with other versions of Tableau Desktop?
Thank you for replying, Mavis.
I confirmed this issue appear in other version, through 2019.1 - 2019.3.
I have reported this issue to support team. I'll wait for the problem to be resolved.