Popular software used in research is incorrectly reporting solution is optimal: what to do?
You should create a reproducible test case that demonstrates the flaw conclusively.  If you are that confident and the software is that widely used, it warrants publication in a technical journal. They key is to involve the company. Show them your test case and mention you have interest in publishing the result. Offer to collaborate with them on the publication. That way, they will be forced to either demonstrate you are wrong or admit you are right. In the latter case, they save face by admitting the problem. If they ignore you, press ahead.