I just check and yes i'm in UEFI mode as you can see on this picture when I boot in UEFI mode with my 8Gb USB stick and my 256Gb USB SSD only the USB stick appears and if I boot on it ventoy tell me that i'm in UEFI. If I change the UEFI mode to legacy I can see both of my USB drives and if I select my SSD (JMicron) it works but i'm in BIOS mode
With SSD and USB stick in Legacy mode (can boot on both) :
With SSD and USB stick in UEFI mode (can boot only on USB stick) :
With SSD and USB stick in Legacy mode (can boot on both) :
With SSD and USB stick in UEFI mode (can boot only on USB stick) :