I use the same user to log on the same system, however some times the Adapt UI button is available but some times not. This behavior is very weird.

See Shell source code for answer.

The "Adapt UI" button is only available if shell is working under "app" mode, that is, an Fiori application is opened currently.

however, when you directly click home button, the shell is working on home mode, as a result the Adapt UI button will be invisible, this is working as designed.

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