So let me get this straight, the only “PXE Reboots after…” is happening on the VirtualBox VM?
I am aware of issues with Virtual Box, specifically, and the only fix that I found (if you’re using undionly) is to do a power off, then power on. The other, more appropriate, fix is to use ipxe.pxe file rather than the undionly, but i also know that not all Physical systems will work with the ipxe.pxe.