I finally found what was the problem.
Thanks go to @bwidyasanyata and Enable Automatic Certificate Generation is fail on Maple after Lilac upgrade - #6 by bwidyasanyata
Unfortunately that causes a much bigger challenge if what he writes is true about courses created before Maple.
I confirm that even importing an old course created before Maple doesn’t work.
How do we handle courses that have been existing for a while, for example on Koa in Native, when we will switch to Tutor and Maple? We have long running courses that deliver certificates either auto-generated or at the end. Is there a migration command we need to run? We can’t assume a course will end before our move to Tutor. That would be a logistic nightmare…