Good it worked for you so far! ?
I can imagine VirtualBox source ebuild or the installed guest additions have some issues. As said, I remember having problems with the source package on my Intel system too. But I have not investigated any further yet due to lack of time. I just switched to the binary package and that worked for me. So unfortunately I can not tell if this is a Threadripper related problem. I hope someone else can help you better than me. Perhaps you should consider writing a bug report on the Funtoo bug tracker. By the end of year I will also retry installing from sources.