Oh actually I found the proble, it's because of a broken no plumbob override that I used. I totally forgot about the existence of the plumbob and forgot that it's also a UI. Nevertheless the problem is now resolved!!
Alicia210899 Please remove GShade and all lighting-related mods or tweaks, and repair the game (EA App > Sims 4 > Manage > Repair OR Steam > Sims 4 > Properties > Installed Files > Verify integrity of game files). Test again, and if you still see the flickering, stop here and provide a dxdiag.
Click Windows key-R, enter dxdiag in the run box, wait for the scan to finish, click "Save all information," and save the file to your desktop. From there, you can attach it to a reply using the paper clip (Attachment) icon included with the other formatting buttons.
If the flickering is gone with default lighting, add one thing back at a time until the flickering reappears. Whatever you added last may be the problem, or it may simply conflict with something else. If you suspect the latter, repeat the process, except test the one identified culprit by itself. If you don't see any flickering, test GShade and your other lighting tweaks against this culprit until you find the conflict.
I would suggest testing with Post Processing off because it doesn't always work the way it should. But if you know you're always going to play with it on, it's fine to leave it on for testing too.
