Debian – UEFI iPXE boot into debian results in grub shell

For a bit of context, I'm a sysadmin student, trying to expand my network knowledge since it's not my strongest point.

Currently, I'm experimenting with PXE. A few days ago, I set up a few ESXi Virtual Machines, one being a DHCP and TFTP server, and the other two being BIOS and UEFI clients. I managed to boot a debian through PXE, on both BIOS and UEFI machines.

I heard about iPXE and the possibilities it brings (such as booting on HTTP, iSCSI, …) and decided to give it a go. For now, I'm still trying to boot my local debian netboot installer, I thought I'd test out remote files after making sure the installation was correct.

At this point, this is what my DHCP conf looks like (minus anything not really of interest) :

option arch code 93 = unsigned integer 16;
class "pxe-clients" {
    match if substring (option vendor-class-identifier, 0, 9) = "PXEClient";
    option tftp-server-name "192.168.0.5";
    next-server 192.168.0.5;
    if option arch = 00:00 {
        if exists user-class and option user-class = "iPXE" {
            filename = "pxelinux.0";
        } else {
            filename = "undionly.kpxe";
        }
    } elsif option arch = 00:02 or option arch = 00:06 or option arch = 00:07 or option arch = 00:08 or option arch = 00:09 {
        if exists user-class and option user-class = "iPXE" {
            filename = "bootnetx64.efi";
        } else {
            filename = "ipxe.efi";
        }
    } else {
        filename = "UNKNOWN_VCI";
    }
}

192.168.0.5 is the LAN IP of the DHCP/TFTP server.

As for my TFTP server's root directory :

root@vmpxe:/srv/tftp# ls -l
total 968
lrwxrwxrwx 1 root root   37 Sep 22 14:01 bootnetx64.efi -> debian-installer/amd64/bootnetx64.efi
drwxrwxr-x 3 root root 4.0K Sep 22 00:21 debian-installer
-rw-r--r-- 1 root root 896K Sep 22 16:00 ipxe.efi
lrwxrwxrwx 1 root root   47 Sep 22 00:21 ldlinux.c32 -> debian-installer/amd64/boot-screens/ldlinux.c32
lrwxrwxrwx 1 root root   33 Sep 22 00:21 pxelinux.0 -> debian-installer/amd64/pxelinux.0
lrwxrwxrwx 1 root root   35 Sep 22 00:21 pxelinux.cfg -> debian-installer/amd64/pxelinux.cfg
-rw-r--r-- 1 root root  64K Sep 22 16:00 undionly.kpxe
-rw-rw-r-- 1 root root   64 Sep 22 00:21 version.info

Again, I insist on the fact that before iPXE everything worked fine on both architectures.
The only changes I made were to download the undionly.kpxe and ipxe.efi files from iPXE's website, and edit the DHCP conf to wrap the filename settings in the if exists user-class and option user-class = "iPXE" {...} else {...} conditions.

Now, the BIOS machine does PXE -> iPXE -> Debian's netboot installer. Everything works fine.

However, the UEFI machine does PXE -> iPXE -> grub shell.

I tried to tcpdump the process but that yielded nothing: all the traffic appears normal and expected. It does not differ greatly from what I had before iPXE.

Any idea what might be happening here ?

EDIT : Further tcpdump testing show that without iPXE, my EFI machine sends RRQ packets for the bootloader and a bunch of other files which, I assume, are referenced by the bootloader itself. However, with iPXE, only the bootloader I only see the bootloader.

So I guess the correct question is : what happens with iPXE for it not to retrieve all those files that are correctly downloaded by a standard PXE ?