You said 'so it boots from the MBR, doesn't need EFI partition for booting.'
If you make a fresh Ventoy USB drive, then partition 1 is completely empty - but you can Legacy boot from the USB drive to the Ventoy menu (which then reboots because the partition 1 is empty). The Legacy boot files are on Partition 2. It DOES need the EFI partition for Legacy booting.
Please can you add screenshots of standard Ventoy USB drive with the two Windows files in the Ventoy menu.
Then show screenshot of UEFI-booting 'good' ISO and screenshot of booting 'bad' ISO.
Also, try formatting first partition as NTFS.
I can boot the ISO OK (partition 1 = NTFS, older Ventoy than you).
Another explanation is that NUC is causing an issue. Can you boot on different systems as a test?
If you make a fresh Ventoy USB drive, then partition 1 is completely empty - but you can Legacy boot from the USB drive to the Ventoy menu (which then reboots because the partition 1 is empty). The Legacy boot files are on Partition 2. It DOES need the EFI partition for Legacy booting.
Please can you add screenshots of standard Ventoy USB drive with the two Windows files in the Ventoy menu.
Then show screenshot of UEFI-booting 'good' ISO and screenshot of booting 'bad' ISO.
Also, try formatting first partition as NTFS.
I can boot the ISO OK (partition 1 = NTFS, older Ventoy than you).
Another explanation is that NUC is causing an issue. Can you boot on different systems as a test?