Hello Insider,
After installed Windows 10 Insider Preview 16251.0 (rs3_release) the lock screen is a solid blue (if I change my theme color, the lock screen changes too). However, I have Windows Spotlight turned on. I've tried setting a static picture or slideshow as the background. I also confirmed that the files needed for Windows Spotlight are indeed downloaded onto my computer. It just refuses to change. Before the update the login screen showed the selected picture.
Thanks