Did you ever find a solution to this? I'm experiencing something similar :(
Edit: I fixed it!! I have no idea what caused the issue or why this fix worked, but it did! The only difference in the issue I was having compared to the one you were having was that Sims WAS registering one of my saves, but it was ANCIENT. The save it was registering was the "SAVE File" type and was named appropriately - "Slot_00000004.save", but the "SAVE File" of my most recent saves were named as though they were back ups, "Slot_00000006.save-DESKTOP-T90SKGT" for me. I changed them to match the former, so "Slot_00000006.save", AND IT WORKED!
DISCLAIMER: I am NOT a computer savvy girlie, this was a total shot in the dark after hours of distress that just happened to work, but I wanted to share just in case anyone is moments away from embracing their clean slate and wants a hail Mary!
