Had the same issue and on my site I installed and activated the Disable Gutenberg plugin (https://wordpress.org/plugins/disable-gutenberg/) and was then able to re-enable the toggle to password protect child pages.
Hope that fix works for others as well!
Thank you for your feedback.
Yeah the site seems to be back to normal now. Very odd… I didn’t take any actions that would’ve corrected it.. Maybe it was just a weird caching issue.
Either way, this seems to be resolved now. Thank you for taking a look at it and providing feedback.
