That is interesting, I saw that behaviour in my Alfa described as: The alternator charges the battery to 85% and then cuts out to save load and hence fuel. If the ECU sees that you are engine breaking, it turns the alternator back on to gain some charge until you are at 100%. So they push it as an economy feature, perhaps because you expect performance from an Alfa but my cars have often drawn comments like "that must drink fuel" and people look surprised when you point out that performance through efficiency means that no, it really doesn't
I bought my car as a three year old car so just out of warranty, and in the couple of months I have had it the stop-start has never kicked in but that just means I don't have to press the disable button so I have been OK with that. The forums say it is always the battery either on the way out or replaced with the wrong type.