Well, maybe I am totally misunderstanding what McMahon means. I have may information from: https://senseis.xmp.net/?McMahonPairing
Before the First Round
Order the field of players by strength. Divide the field into bands of players based upon number of rounds and the distribution of players strengths. Players in the same band will be paired against each other in the first round. There are several ways to do this, for details refer to the section on assigning initial bands. The highest group is generally referred to as the “top group”, the lowest as the “bottom group”.
Assign starting McMahon Scores
There are two main ways this is done. The first is to set the McMahon Score of the top group to 0, with all other bands starting at a negative score. So the next weaker band is -1, down to the weakest band which is -(M-1) for M bands. The other is is to set the McMahon Score of the bottom group to 0, with all other bands receiving a positive initial score. In effect, the early slaughter rounds of the Swiss pairing are replaced by assigning a number of virtual won games to stronger players. The stronger the player the greater the number of won virtual games assigned to that player.
Note that this is nowhere talking about simultaneous McMahon.
I think what is described in senseis is not at all what is happening in our tournament.
There may be 4 possibilities:
I am reading it wrong and it actually is what is happening in the tournament. In that case, could someone please help me understand?
What is written in senseis is just wrong. It should be changed there.
There are many different definitions of McMahon and the one on OGS happens to differ from that on senseis.
There is a bug.