It could be part of a shadow mesh issue, but to me it looks like it's to do with SSAO (which is ambient occlusion - part of the lighting settings).
You can turn that off by editing the game's graphic rules, which is a settings file. Here's how:
Navigate to your game's Origin installation directory (not the folder where you have saves, mods, screenshots, etc). For me, on a Windows 10 PC, it's here:
Program Files (x86)/Origin Games/The Sims 4/Game/Bin
In this folder, there's a file called
GraphicRules.sgr. You should copy that to somewhere safe on your computer before you make any changes to it, just in case!
You can open the rules file with pretty much any text editor (my computer defaults to Notepad) and do a search for "SsaoEnabled". The game sets whether SSAO is enabled or not for each lighting quality, from low to very high, so you can either change all of them or just change the one that matches the light setting you use in game. For example, I use very high, so I find the "SsaoEnabled" line under "setting $VeryHigh".
Next to "SsaoEnabled", it says true. Change that to false, save your file, and you're good to go.
If this is what's going on in your game, I hope this helps :smile: