It seems the character designer for the game is Youichi Kodama, who doesn't appear to have worked on Bahamut Lagoon. He doesn't seem to be terribly prolific with The Glory of Heracles IV being his most notable credit from MobyGames. So the answer to your suspicion is that he probably liked the look of Bahamut Lagoon and copied the style for Valhollian. I certainly like how the game looks.
Glory of Heracles IV and Bahamut Lagoon have the same director, interestingly. He was hired away from Data East by Square for his work on that series.
Also, depending on the company, the character designer might not be responsible for the actual sprites. They might only draw character illustrations, and then other people make sprites based on those. Companies often used uncredited freelancers, and many old school Japanese game creators used multiple aliases (and not just obvious nicknames). All this to say that staff overlap is not entirely impossible.
Congrats on the release, btw. I plan to give the game a go soon.