I guess this is because Polyphone is not a known editor for Bill Gates and by default he considers everything is possibly a malware You can safely install Polyphone. Click on "more info" and then "run anyway".
As much as I would like to believe your assurances, this was a Malwarebytes report and had nothing to do with Microsoft. The proper route is for you to send this to Malwarebytes for analysis and verification.
The checksum of the file I uploaded matches the checksum of the file I downloaded from the website so the file is OK. There is no virus in what I provide except if my PC have been hacked but this would be an unlikely scenario. I would think of a false detection