Got the same machine. HP Compaq 8000 Elite.
Had same issue. Won't boot from multiple partition in MBR.
Found out that BIOS can be updated. After refresh the BIOS below, now this HP Compaq 8000 Elite machine can easily boot with at least "DS3615xs_V1.03b_MBR_Zandstraal.img" file.
https://support.hp.com/us-en/drivers/selfservice/swdetails/hp-compaq-8000-elite-small-form-factor-pc/4065894/swItemId/vc-156854-2