SUMMARY
Setting Picture of the day does not work for lockscreen
Looks like this is a long-standing issue, but I haven't found any bug reports here
It was discussed on several websites:
https://www.reddit.com/r/kde/comments/711lej/picture_of_the_day_lock_screen_background_not/
https://www.kubuntuforums.net/showthread.php/74793-Lockscreen-set-to-picture-of-the-day-yet-never-shows-only-clock

STEPS TO REPRODUCE
1. Open System Settings, go to Workspace Behavior -> Screen Locking -> Appearance
2. Set Wallpaper type to "Picture of the day"
3. Lock the session

OBSERVED RESULT
Lockscreen background shows only solid color

EXPECTED RESULT
It should show the Picture of the day

SOFTWARE/OS VERSIONS
Operating System: KDE neon 5.19
KDE Plasma Version: 5.19.2
KDE Frameworks Version: 5.71.0
Qt Version: 5.14.2
Kernel Version: 5.3.0-59-generic
OS Type: 64-bit