Dock disappears and reappears on next reboot in Mac OS X Catalina.

This happened to me too. Recommended fixes did not work. I found this solution worked permanently:

1. Got to System Preferences > Keyboard > Shortcuts > Launchpad & Dock
2. Turn off 'Turn Dock Hiding On/Off'
3. Go to System Preferences > Dock
4. Untick 'Automatically hide and the show Dock'

If this does not work, then go back to Systems Preferences and reset the settings above as they were before making changes. Then restart and make the same changes listed above but in this order:

1. Go to System Preferences > Dock
2. Untick 'Automatically hide and the show Dock'
3. Got to System Preferences > Keyboard > Shortcuts > Launchpad & Dock
4. Turn off 'Turn Dock Hiding On/Off'

Good luck!