Looks like a case described in PMD docs (https://doc.dpdk.org/guides/nics/mlx5.html):
If Multi-Packet Rx queue is configured (mprq_en) and Rx CQE compression
is enabled (rxq_cqe_comp_en) at the same time, RSS hash result is not
fully supported. Some Rx packets may not have RTE_MBUF_F_RX_RSS_HASH.
I recommend checking that page in general, because mlx5 PMD supports many
features, but not all combinations of them.
