Comment 1 for bug 1806961

Thanks for taking the time to report this bug and helping to make Ubuntu better.
LightDM has an internal tool called dm-tool which provides some features for minimalist
window managers. Although it provides this tool, because of the many locking issues
it is not recommended to use it. So an alternative screen-locker application is necessary
to provide better security, one of them is light-locker, but there are many others.

Could you please install light-locker and test if the issue continues?
sudo apt install light-locker

For more information on light-locker:
https://github.com/the-cavalry/light-locker

If you still would like to see this implemented on LightDM, I would recommend going to the link
below and asking on the issues section. Please understand that this might never be implemented
by the developers and they might comment why.
https://github.com/CanonicalLtd/lightdm