Replace "Ubuntu Dock" from extensions list with "Dash to Dock"
When you have the extension "Ubuntu Dock" installed from the Gnome-Tweaks tool, locking the screen doesn't work correctly. This can be resolved by simply using the "Dash to Dock" extension instead.
More info
gnome-shell#794 (closed) https://bugs.launchpad.net/ubuntu/+source/gdm3/+bug/1803807?comments=all
This resulted in several annoying days of trying to figure out what was going on, and I would have to restart my computer in order to fix the locked screen issue.
Edited by Ghost User