Submitting on behalf of Anthony Vital, who contributed the patch at https://bugs.kde.org/show_bug.cgi?id=482306#c12. I tested it on my ThinkPad and can confirm that it fixes the issue.
CC @nicolasfella whose refactoring is what made the condition disappear - please confirm that the removal was accidental during the keyboard/screen brightness split.
Out of the two checks in this line that were removed incommit d0b43050, one should have remained. We still need to makestoring the old keyboard brightness conditional on an opened lid,otherwise restoring keyboard brightness can fail after closing andre-opening the lid.