Comment 2 for bug 1854839

I may have found a workaround/fix for this. I noticed that /etc/grub.d/30-os_prober has a GRUB_DISABLE_OS_PROBER variable that will skip probing for other OSes on the machine, so I added GRUB_DISABLE_OS_PROBER=true to the kernel options and the installer proceeded to completion on a machine that was consistently hanging at 66%/osprober. I'm using foreman for the provisioning so my pxegrub2 template looks like this now:

menuentry '<%= template_name %>_uefi' {
  linuxefi <%= @kernel %> auto=true netcfg/choose_interface=<%= @host.primary_interface.identifier %>np0 url=<%= foreman_url('provision')%> ramdisk_size=10800 root=/dev/rd/0 rw auto hostname=<%= @host.name %> GRUB_DISABLE_OS_PROBER=true <%= options %>
  initrdefi <%= @initrd %>
}

As soon as I added this option, the install completed. Have only tested it on this single box, will update again if this doesn't fix it on future machines.