Hi,
In this case, could you name the park?
Maybe there are other records of the same bird in ebird or inaturalist, with even the bill showing maybe.
It seems too light mantled for Pochard x Ferruginous, but that is probably within range of the options for this hybrid.
edit: this brown Pochard x Ferruginous showed up less than a week ago:
https://ebird.org/japan/checklist/S157200874
cheers,
G