Iruma have yet to fail the test to rank up to rank 6
A quick reminder that iruma did not fail, his result are being evaluated. At heart, the purpose of the exam is that anyone who takes the exam must do something that benefits the netherworld (or society, to be exact).
The 13 crowns overseeing the exam can design a test under that spirit and set their own passing goal.
Iruma did pass mephisto's test perfectly, however, the main "benefit to the world" point is that he should be setting a stable "source" of the many eared people to export workers since that has been their job since forever.
Iruma did make a nation (which is mephisto's test) and certainly guarantees the population of the many ear. However, by creating a whole new industry for them to work, less of them will be working jobs like before, such as jewelry examiner, music professionals, and reconnaissance in battles. In return, they're making high-quality food, which is also a problem to be tackled but at the price of other jobs.
They're evaluating whether the benefit of that is greater than them doing their traditional jobs and whether the increase in population will make up for less percentage of their race doing the job, which is a few more generations to come. For that reason, iruma's rank up must be evaluated further.
So we might not have another scala but rather just a chapter to announce his rank up (or they might fail him)