Query engines are (not)surprisingly complex software products. Add to that the constant (and aggressive, due to the competition in the field) evolution and adition of the new features that can interact with every existing feature in any existing context and you have a perfect environment for bugs to appear.
Oracle might be military grade because they have the entire web page on how to report wrong results bugs: https://support.oracle.com/knowledge/Oracle%20Cloud/150895_1... https://support.oracle.com/knowledge/Oracle%20Database%20Pro... etc.
Query engines are (not)surprisingly complex software products. Add to that the constant (and aggressive, due to the competition in the field) evolution and adition of the new features that can interact with every existing feature in any existing context and you have a perfect environment for bugs to appear.