EDIT: as wankingSkeever points out below, the simple restart of the game could have fixed a transient issue. The shaders did rebuild before those screenshots were taken, and of course rebuilt again after I disabled NSTM. Now I can't say this is the cause until / unless someone else duplicates the outcome.
https://www.nexusmods.com/skyrimspecialedition/mods/45401 I disabled this mod and the problem went away.
https://i.postimg.cc/Z5NQ9pkG/Skyrim-TOOBRIGHT.png
https://i.postimg.cc/t46c1W0L/Skyrim-TOOBRIGHT2.png
So, if you update CS and get what I got in those images, look to see if you've had NSTM installed and forgotten about it.
MORE EDIT: as you can see, I don't know the difference between Markdown and Rich Text or how to format a link to an image.