If it's a proven fact that they can't be happy living alone, then I'm sorry, but your experience isn't an exception.

I was worried the boy rehomed to me wasn't going to be able to re integrate back into a mischief because he was kept alone for about 2 ish months before I got him. I made sure his quarantine cage was very crowded and enriching but he didn't really interact with them much compared to my other rats who were all adopted either together or introduced at a very young age. Depressed rats act like I'll rats. They'll be more lethargic and don't exhibit common rat behaviors. Then he had to wait another 5 weeks before he was 100% sterile to be introduced to some intact girls as he just couldn't get along with my only other boy (recently lost his brother/only cagemate but ended up with 2 spayed girls)