Why does meat go bad after 1 month sous vide at 55-60C ( 131 - 141F)
40-140F is the danger zone of high growth rate for bacteria.  60-120F is the extremely high zone.  55C is only 131F and not even out of the high growth rate area, 60C it at the edge right at 140.



Outside of the high growth area bacteria growth is not zero, it is only much lower and there is not a magic switch that instantly kills all bacteria when you hit 60C.  The safety rules say that at that point you have reduced risk, not zero risk.  Bacterial growth certainly continues simply at a lower rate.  However, a lower rate over longer time still constitutes rot and danger.  The safety provided by reaching a prescribed temp is negated by then trying to stay there for an unreasonable time.  No guides that say to reach that temperature for safety state that all bacteria are killed, only that many are and those that survive will not be reproducing at rates to be a likely danger if you otherwise handle the item appropriately.  Waiting another 30 days is not handling it appropriately.

In addition, there is more to decay of a piece of meat than just microbial action.  You have cell structure breaking down, chemicals being released, etc.  I would fully expect, in fact be shocked, if the brisket in question would not effectively begin the equivalent of digesting itself.  Bad smells and a texture approaching jelly would not be surprising in the least.  A potentially interesting science experiment, but nothing resembling food should come from any such tests.