Convert u256 to string

what I am trying to do is something similar to this: Convert u64 to String for concatenation - #2 by Serafim

I am using the same code, the difference is that this guy is converting u64 to String and I am trying to convert u256 to String. Wouldn’t he loose some bits of precision as well? But it seems to be working there. So, I am not sure why is it not working for u256