FS Q-8LCA020 QSFP+ to 8xLC breakout AOC never links on a ConnectX-4 MCX414-GCAT port

We are wiring an FPGA capture card into an existing server and I am stuck at the very first step: the port simply never comes up.

The setup:

• Mellanox MCX414-GCAT (ConnectX-4 EN, QSFP28, 50GbE capable) in the host
• FS Q-8LCA020 breakout AOC, QSFP+ on one end, eight LC ends on the other
• FPGA card on the far side taking the LC legs

The host is happy to power the cable and read it, but the link never establishes:

# ethtool ens2f0
        Link detected: no

What I have already done:

• moved the AOC to the second port of the same MCX414-GCAT, identical result
• read the module back with mlxlink on the mst device: the cable identifies itself as the Q-8LCA020 and nothing in the module state points at a fault
• reseated both ends and double checked that the LC legs land on the lanes the FPGA design expects

What I cannot judge from here is whether this combination is supposed to work at all. It is a QSFP+ class breakout AOC in a QSFP28 port on a NIC that would rather run the port at 50G, with an FPGA instead of a switch on the far end, so there is no switch side to compare against.

Has anyone brought up a Q-8LCA020 on a ConnectX-4? And if the cable is simply outside what the NIC is willing to drive, is there a way to establish that from the NIC side rather than by swapping hardware until something links?