Since WordPress 5.4, the block editor has opened to the fullscreen mode by default. Since this information is stored only as a local setting, this will happen for users every time they open the editor on a device where they haven’t previously configured their preferences, if they reset their local storage, or when setting up a new site.

Enabling this feature in WP Accessibility still allows you to choose full screen editing as your preference, but it will not be the default.

Since WordPress 6.1, block editor settings such as dismissing the welcome modal, disabling fullscreen mode, and other equivalent settings are saved in user metadata, and are no longer dependent on local storage.

Settings panel with option to disable the full screen block editor default.