Would just like to add to this - I recently ran into this same issue again. Racked my brain trying to figure out what I did wrong or what setting could have possibly changed. Here’s what I found out.
I was using the above PC again after cleaning and prepping to capture another image. This PC was already registered in FOG. I deleted the host and attempted to PXE boot again and everything went smooth as butter. Looks like there’s an issue booting if the device is already registered. I’m not sure what the issue is, but deleting the host and re-registering solved it.