Functional hallux limitus is a theoretical construct. There is no doubt that the phenomenon that is functional hallux limitus exists, but that can often be as far any any agreement or consensus gets with this. There is no doubt that there are some feet when the foot is up in the air that there is a full range of motion available at the the first metatarsophalangeal joint yet when that foot is weightbearing during gait, that joint just does not want to move. That is the definition of what functional hallux limitus is.
I have written about this a lot, speculating that functional hallux limitus has many possible causes and that it may be better to conceptualize it as a nothing more that each person having a variable stiffness range of motion at the first metatarsophalangeal joint and that this may be related to the windlass mechanism. If this is the case then what we typically considered to be a functional hallux limitus is really the extreme of this variable stiffness and that functional hallux limitus probably exists as a continuum and not as an either/or entity.