In order to use Bhyve with passthru you need to go into the config menu and enable iommu and save. That option seems to be available with https://pcengines.github.io/#mr-30 of the firmware.
In addition, give this is AMD you need to enable a tunable according to the end of this page.
Here is an example of what I use for 2 mPCIe cards and igb1/2:
# Turn on passthru support on AMD; # see bottom of https://wiki.freebsd.org/bhyve/pci_passthru hw.vmm.amdvi.enable=1 vmm_load="YES" pptdevs="3/0/0 1/0/0 4/0/0 5/0/0"