@Matthieu-Jacquart Seams like we are pushing this quite far. Michael from iPXE now contacted people at HP to see what they might know about this. Might be a firmware bug. We will see.
From the things I posted the ipxe.efi binary with more dump code is the most important! This might be important information. Please get into this first.
And there is one more thing that just came to my mind. Have you tried all these things on one of your tablets only? Could you please do the USB stick test on at least two or three other devices to make sure we see the same issue and address/size values on all of them!!!