For anyone coming across this post, before you try to any of the other options which didn't work for me (uefi/pci-e mode, disabling boot on card, setting romfile manually) or flashing the card, try setting rombar=0. This presevents the guest from seeing the firmware rom file, which did as it said and fixed this issue for me. You can even set this option from the proxmox UI by enabling advanced options and ticking the box.