@matthew73 This is a unique condition. I can understand what is going on because we use NAC and VLAN switching on my campus. I can say that I have not seen this issue (anywhere) on my campus.
I think I understand what needs to happen. Basically iPXE needs to say something and then wait XX seconds for your NAC system to identify the hardware and to switch it to the right vlan. The network link light winking happens 2 times during a normal pxe booting. The first time is when the PXE turns over control of the network adapter to iPXE and then when iPXE turns over control of the network adapter to FOS Linux. We see a similar issue when the network switches are using standard spanning tree and not one of the fast protocols (RSTP,MSTP, port-fast).
The developers have created a specific group of iPXE boot loaders that have an embedded 10 second delay before iPXE tries to request an IP address. This gives STP and powersaver functions on the switch a chance to react before iPXE starts to talk. These files are in the 10secdelay folder. So to use these update dhcp option from ipxe.efi to 10secdelay/ipxe.efi This will call in the 10 second delay boot loader. See if that makes things better or not.