It could also be cultural differences, a selection bias in the participants, differing criteria for "success/productivity"... or a thousand other reasons, too. It's easy to jump to a conclusion or speculate why a desired outcome wasn't the actual outcome; but the best you can probably say is you just don't know why one was a "success" and the other a "failure" and look for a study of the reasons for a given outcome.

I find it rare that popular reporting gets to the nuance required to really understand a research outcome much less to ascribe plausible, grounded reasons for that outcome.

French here, working/living in France, and not aware of any state-backed “shorter work week” test. According to my research, only SME applied it so far (with 2 exceptions of 1000 and 11k-employees). And no particular consensus on the results (many companies stayed on their 4d agenda after the initial phases, but again, we’re talking SME here).