Yes I think this is the problem with this Acer machine. I cannot boot any non-UEFI devices if the BIOS is set for UEFI. I enabled the BIOS boot selection menu, and when I'm in UEFI mode, the SSD is listed, but the DVD drive or any other USB stick that is MBR does not appear. As soon as I put the UEFI formatted USB stick in, I can see it appear in the boot selection menu. If I switch to legacy mode, all the legacy boot devices will appear and I can boot from them, but I cannot boot from the SSD.
It seems this Acer strictly only boots UEFI devices in UEFI mode, and legacy devices in legacy mode, which I agree seems very unusual.
And yes I've got secure boot disabled.