I am unable to recreate this myself in VMs... Michael may have other ideas too.
So from Comment #8 above, what image are you using, exactly to attempt this?
and further questions:
And are you sure you're using the 18.04 netboot images?
How exactly (what steps did you take) to set up TFTP to serve the netboot images.
How exactly (what steps did you take) to set up the ISO image for PXE/Network installation?
I am unable to recreate this myself in VMs... Michael may have other ideas too.
So from Comment #8 above, what image are you using, exactly to attempt this?
and further questions:
And are you sure you're using the 18.04 netboot images?
How exactly (what steps did you take) to set up TFTP to serve the netboot images.
How exactly (what steps did you take) to set up the ISO image for PXE/Network installation?