Actually what you observe is a consequence of the OP, and it’s how I was made aware of it from https://github.com/JuliaLang/julia/pull/36470#issuecomment-703036745:
125/126> rand(UInt256)
0x00000000000000000000000000000000000000000000000000000000000000d2
127> string(big(ans), base=16)
"28fecc917ccc6d1bbf9b54c500c26d0c8dea1d23520b8d4d87e9377ddf6099d2"