It is incredibly common to the point it makes no sense to make any kind of bets on small events as you are at the mercy of those that want to rig the sport, I like boxing and I even go and watch new fighters, or at least I did before the pandemic came, and at least once every night there was a moment in which I said this fight is rigged, I could clearly see one of the fighters not giving it all or the judges giving the fight to someone that did not deserve it and that is extremely disappointing for those that actually love the sport since it destroys the incentive to keep watching it.
How common are such incidents in the sport of boxing? Such fixing may occur in lower tier fights. But there is zero chance that the top fights are being fixed. That said, the scoring has been a contentious issue for sometime in boxing. I have seen a number of instances when the win was awarded to the boxer who performed poorly.
A good example is the (first) fight between Canelo Álvarez and Gennady Golovkin. Adalaide Byrd had a shocking scorecard of 118-110 for Alvarez (meaning that he won 10 out of the 12 rounds) when the TV broadcasters and neutral observers claimed that Golovkin indeed won 8 to 10 rounds. And the biggest joke is that Adalaide Byrd continues to be a judge. But I don't have any reason to believe that Byrd indulged in fixing here. The biased scorecard possibily resulted from the personal bias towards Álvarez, rather than due to any monetary considerations.
Yes, I was talking about low tier fights and at least to me it seems this happens quite often but like always circumstances change, it could be very common where I live but not as common on many other different countries, but it is undeniable that it should be way harder when it comes to professional fights and yet it probably happens from time to time, I remember the fight you are talking about, and I remember scoring that fight 113-115 for GGG, I did not mind the judge giving 114-114 as the fight was close and it is within the margin of error, but the judge that scored the fight 118-110 saw a different fight than the rest of the world.