Other testbeds with Mellanox-CX7VEAT (or cx6) NIC have no problem configuring MTU of 9200 (even for RDMA core driver, although that still fails on zero traffic due to CSIT-1906).
Only 3na-spr is problematic, so this may be an infra issue, not a VPP bug.
For 3na-spr and mlx5 tests, 3502 is the largest MTU not refused by VPP call, which is of course too small for 9000B tests, but may help identify the cause.