I've heard a lot of the same from the professors I'm close to, even when taking significant measures to prevent or reduce the incentive to cheat (like making the exam less important, making it open internet but no communication or using that annoyingly intrusive lockdown browser). Even when having the ability to look things up on the internet, there were cases of it being relatively obvious when people were communicating (not that I know how).

Many of them just ended up cancelling the results for those exams outright.