Hi,
My lock screen kept changing back to the ventura one but I fixed this issue by going into my Hard Drive : /library/caches/Desktop Pictures/
I had the Ventura desktop picture.png in this folder for some reason so I deleted it, since then everything has been okay.
Not saying it will work for you, but it fixed my issue.
On further investigation, the ventura screen reappeared, the issue is resolved by selecting the screensaver system prefs and choosing both the screensaver you want and check the "use as desktop" setting, but don't open the wallpaper system prefs and select from there the wallpaper and check the "use as screensaver"setting, that is what is causing the conflict.
Derrick1