For the analogue of the legacy cli-shell-api completion, it is required to refer to the help element in the absence of a valueHelp element. For the integration of the vyconf backend with the existing bash completion, we are simply reproducing the behavior of cli-shell-api, which can be traced to this case:
https://github.com/vyos/vyatta-cfg/blob/current/src/cstore/cstore.cpp#L864-L868
Though the case had been identified, it was missed in the initial version of the compatibility tools (T8061).