Partition sizes are always multiples of 512 byte sectors. In fact, sectors
in computing are almost always 512 byte units, even when the underlying
medium uses larger physical sector sizes (e.g. modern HDDs, Flash/SSDs).
The only exceptions I can tell off-hand are physical CD sectors and old
MO drives’.

Short of the HP “BIOS” special-handling a partition type 4, I cease to
have any ideas besides the obvious “it’s ISOLINUX’ fault”.

As another possibility, copy the entire first 32 KiB from the test.iso
over these of the grml ISO, thus replacing MBR/partition table (with
smaller values, but we don’t care here, for now) and GRUB2.