As I’m trying to understand a bit further,
Your tablets are connected to Docking stations with LAN ports built in. Do these docking stations have PXE/GPXE Support for them? When you boot the system up and get into the bios, does it give options for PXE booting with and/or without the Docking station connected?
Is there a possibility of multiple areas in your BIOS for PXE booting?
The last thing I can think of,
Are the tables UEFI enabled? If their UEFI enabled, is it Security Enabled as well? If they are, have you tried disabling UEFI on the system and then PXE booting? I have about 200 systems that came in for this year with UEFI enabled, but security was disabled and legacy mode was enabled. I didn’t have to disable UEFI and all worked fine. I’ll see if I can test the system with Security enabled and pxe boot. My systems are not tablets, they’re regular desktops, but it’s about as close to troubleshooting as I can get.
I’m sorry if you’ve already tried all of these things.