There really isn't any diminishing of his force connection from the end of Son of Dathomir up until however he got onto Malachor. Given Filoni stated Vader was in his prime in Rebels, and the most current version of Maul outright admitted he couldnt take Vader on his own I'm not seeing how Maul wins.
__________________ "Happiness is a lie. Life is horror. The light is always dying all across the universe. The last star will flicker out someday, when it does, all that remains is shadow. And I will be its king!"'-Amahl Farouk
He still looked physically able to me, and reverted back to his saber staff which is a weapon that is notoriously difficult to wield. Apparently he also fought Ahsoka before, and given his penchant to train, exercise to the point of insanity, he's obviously not in any sort of physically degraded state
__________________ "Happiness is a lie. Life is horror. The light is always dying all across the universe. The last star will flicker out someday, when it does, all that remains is shadow. And I will be its king!"'-Amahl Farouk