Sorry for the late response, thank you!
Just to close the discussion, in case someone encounter the same issue, I tried to unplug a disk while the system was running: in this case the array was correctly moved to "degradated" state but it was still accessibile (even after a reboot). In the very end, I think the "problem" is the fact that mdadm needs to "record" the changes in the array and can not automagically detect what happened when the system was powered off (such as an unplugged disk).
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