Mac OS Sierra System Settings won't stay set
It’s possible your Accessibility preference file is corrupted.

Open Finder, then press ⇧ shift ⌘ cmd G. In the popup textbox, type in ~/Library/Preferences.

Locate com.apple.universalaccess.plist and delete it. Now restart your system.

If the issue persists, you can delete the new preference file, then pull the original file back out of the Trash before restarting one more time.