One field seems to have the API defined incorrectly:

[

    "vl_api_fib_mpls_label_t",
    "label_stack",
    16
]

 should be: 

[
    "vl_api_fib_mpls_label_t",
    "label_stack",
    0,
    "n_label"
]

This causes our bin api generator (GoVPP) to produce code which cannot be encoded/decoded automatically. We have to manually fix this after every generator execution.