Calls to `MASM_RDSEED_GenerateBlock` would hang for an unknown reasons on Windows 10 and VS2017/VS2019 toolchains. Similar calls to `MASM_RDRAND_GenerateBlock` worked as expected. They were effectively the same code. The only differences were the function names and the opcodes (they were literally copy/paste).
Splitting `rdrand.asm` (with both `RDRAND` and `RDSEED`) into `rdrand.asm` (with `RDRAND`) and `rdseed.asm` (with `RDSEED`) resolved the issue. We don't know why.