Hi Snow Lu,
Just a catch-up with you.
Investigating with our IT staff, turned out only my device has this kind of issue even they reinstalled my Microsoft 365. IT staff suggested that this's not local policy issue.
My solution now is to re-order the QAT bar by arranging all default buttons after my personal setting, so that I won't keep clicking the wrong buttons. Only by doing this I can save the setting.
Again, thanks for your help.