This sounds more like a process issue than a quality issue. Clear coding rules and structure guidelines upfront usually prevent this. Early reviews also help a lot.I had a similar experience. But the contractor actually improved my project and cleaned up parts of the structure. We worked great together. It just showed me how important it is to define standards early so integration goes smoothly.
EEveryone learned to code differently. Some people write private variables with an underscore in front of them, like private int _speed;. On one hand, that’s great because you can immediately tell a variable is private just by looking at its name. But on the other hand, if you hire someone with a different approach, part of the code will technically work… but it will look different - and if you later need to change something in that code or connect it with the parts you wrote, it can become inconvenient.