I forgot that the stay plus night only applies to the lowest room category.


AccorPlus just answered me:

The Stay Plus benefit is applicable to Standard Rooms, which may vary by hotel brand. Some hotels may offer you the chance to upgrade to a higher category room for a nominal fee when utilizing Stay Plus benefits. After examining your reservation, we found that your booking includes a flexible rate for a Deluxe Room, which comes with Stay Plus benefits. This room type is an upgrade from the Superior room, thus a small upgrade fee is necessary for the Stay Plus night.


I should have booked the Superior room (which is the entry level room) to get the fully complimentary room on the price details.