Dear Levkoni,
as my fiancé had the same issue I took a look into that.
Since you, your friend and besides my fiancé several of her friends came to me with the same issue on first glance this seemed to be some kind of graphic bug to me or one way of artificially enlarging the RAM capacity like it was done with some older games on older systems.
Usually the RAM enlargement would make it appear flashing and changing in length but this didn't seem to be the case.
When digging deeper I noticed that there is another slightly darker bar on the right edge aswell.
Also did I notice that the corners where accentuated with an even darker pixel and all three shades of white were the exact same as they appeared on the edges of the Sims4-logo.
Beside that these lines are exactly one pixel in width.
After gathering all this information it seems to be there on purpose to give it a rudimentary 3D-ish look like it was done in early games mainly at the edges of text boxes and dialogue menus.
Only Maxis knows the reason for that but if you can locate the template for the main menu and the loading screen - if there is any - you could use a graphic editing software to alter that to it's previous borderless appearance. Maybe there is already some mod which addresses this.
Sincerely Pudding
