Timeline Zooms out after Full Screen Preview

You want to try resetting preferences:

1. Launch After Effects and hold down any modifier key, such as Command, Option, Shift (macOS), or Ctrl, Alt, or Shift (Windows). Or open it through the Preferences dialog.
2. If you've opened the Reset options dialog by holding down any modifier key, select the relevant options, press the Continue button to perform the specified actions, and resume the launch process.
3. Depending on the keyboard modifier (or modifiers) used, the options will be pre-selected in the dialog:

Opt or Alt: Reset app preferences

Shift: Reset plugin loading cache

Opt or Alt + Shift: Reset app preferences and reset plugin loading cache

Cmd or Ctrl + 3: Disable third-party video plugins

4. If you've opened the Reset options through Preferences, then select your relevant options from the given dialog.
5. After selecting the OK button, a confirmation dialog will appear, displaying the specific options selected in the previous dialog.
6. Select OK to confirm, and the next time After Effects is manually restarted, the selected options in the Reset options dialog will be processed.