>> I hit this problem this week too, did you learn anything more?

If it can help, I change boot loader to GRUB - same problem occurs.
  Booting 'netbsd'
--
Aymeric


root (hd0,a)
 Filesystem type is ffs, partition type 0xa9
kernel --type=netbsd /netbsd
   [NetBSD-elf, <0x100000:0x24edc0:0x0>, <0x34fdc0:0x200d664:0x2939c>, shtab=0x
2387258, entry=0x100000]
þ