Newer Windows systems pretend that anything Microsoft hasn't whitelisted as popular is a virus (it pops up a 'security warning' message that suggests a risk of viruses from using software the megacorporation isn't aware of, which I've seen many less-sophisticated computer users interpret as a virus detection). I don't think there's anything you can do about it without specifically engaging with Microsoft or having the software become very popular, but it's true for most home-made games (e.g. the vast majority of games on itch) not by somebody with a well-established portfolio. If the game becomes popular the problem will fix itself. It's just a way to try and shut down independent development so as to further hegemonize corporate power.
It's possible there's some other thing it triggered for that user; I'm not getting a 'rarely used' security warning after downloading, but I'm not getting any warning at all so I'm guessing that's it.