After some extensive testing I have further information to report that may help solve this issue.
I can confirm that the issue exists in Lubuntu 18.04.2, Lubuntu 18.10, Xubuntu 18.04.2 and Xubuntu 18.10, all of which use light-locker 1.8.0. Of interest a fresh installation of the original Lubuntu 18.04.0, then updated with all the updates available to date does not have this issue. This makes me think that the problem is not light-locker directly, but one of its dependencies that has been updated in the 18.04.2 package, but that was not updated in the normal update process from 18.04.0 to date. If you can identify which dependency it is, then it should be easy to solve this bug.
After some extensive testing I have further information to report that may help solve this issue.
I can confirm that the issue exists in Lubuntu 18.04.2, Lubuntu 18.10, Xubuntu 18.04.2 and Xubuntu 18.10, all of which use light-locker 1.8.0. Of interest a fresh installation of the original Lubuntu 18.04.0, then updated with all the updates available to date does not have this issue. This makes me think that the problem is not light-locker directly, but one of its dependencies that has been updated in the 18.04.2 package, but that was not updated in the normal update process from 18.04.0 to date. If you can identify which dependency it is, then it should be easy to solve this bug.