The new Ryzens are known to contain several bugs, e.g. with their RDRAND instruction. This is not a bug in MKVToolNix, and AMD has released new BIOSes that're supposed to fix some of those bugs to for the motherboard manufacturers. Another user who reported the issue
over on Gitlab also reported that their motherboard manufacturer's just released a BIOS update. After having applied the update MKVToolNix
started to work fine for them.