User switching fails with computer locked if mouse moved to second monitor

[VanillaMozilla]

Ubuntu 18.04 LTS 64 bit. Unity desktop
Two-monitor setup, with extended desktop (not mirrored)

Steps to duplicate
1. Requires two monitors. The computer should have more than one user account, with user logged in to an admin account.
2. From the primary monitor, click on ikon in upper right hand corner. Select Lock/Switch Account...
3. Move mouse to the other monitor, and optionally back again.
4. Click on that ikon again to switch users.

What should happen
A dropdown menu should read:
Switch account...
<user 1>
<user 2>
<etc.>
Suspend

What actually happens
The ikon changes color on clicking, but is otherwise inactive. No menu appears, and there is no way to switch users.

[Ubuntu Foundations Team Bug Bot (crichton)]

Thank you for taking the time to report this bug and helping to make Ubuntu better. It seems that your bug report is not filed about a specific source package though, rather it is just filed against Ubuntu in general. It is important that bug reports be filed about source packages so that people interested in the package can find the bugs about it. You can find some hints about determining what package your bug might be about at https://wiki.ubuntu.com/Bugs/FindRightPackage. You might also ask for help in the #ubuntu-bugs irc channel on Libera.chat.

To change the source package that this bug is filed about visit https://bugs.launchpad.net/ubuntu/+bug/2013007/+editstatus and add the package name in the text box next to the word Package.

[This is an automated message. I apologize if it reached you inappropriately; please just reply to this message indicating so.]

[Daniel van Vugt (vanvugt)]

Ubuntu 18.04 reaches end of standard support in a couple of months so this might be a good reason to install Ubuntu 22.04 instead.

https://wiki.ubuntu.com/Releases