August - last edited August
@Annarata Thank you. Here are some more things to try:
Disable OneDrive if you haven't already.
EDIT: Create yet another new Sims 3 folder, install just one item to it. Check to see if it is installed in game, or in Main Menu if it is a world.
Restart your computer. Check to see if the item is installed or gone.
If it is gone: create yet another folder, install one store item to it.
Toggle the button Run without custom content found in the launcher>Installed Content tab and clearing Internet Explorer's cache, cookies and history: check it, clear IE cache; Go to Main Menu in game; exit game; uncheck the button, then clear IE cache again. Make sure you uncheck preserve favourite website data when clearing IE's cache.
Restart the computer and see if the item is installed/gone.
If it is gone reset IE and see if that makes a difference.
Sorry about the edits.
(Thanks to @puzzlezaddict )