A few thoughts from me.
I do not know if I have the same battery as you in UK, as in Poland they deliver cars with something like 'adoptation for winter' which is not officially explained, but some rumours were that it is stronger battery only.
In my Veloce (manufactured in Apr 2018) I got Varta Heavy Duty battery, 95Ah.
I suffer the same problems as many of you describe. S&S worked for say first month or so after delivery, then stopped. I was not complaining knowing it is standard behaviour of Giulia. Then in Aug I did nearly 2kkm holiday drive, so it went back again for some time and then gone. Problem started in Nov, when rear screed defrosting stopped working. Battery rechargin and all is fine. But, that lead me to start digging a bit. I started checking my battery voltage (bought small voltmeter plugged to cigarette lighter), started connecting PC with MES to check battery state of charge reported by IBS. During drive, I always see 14.4-14.6V charging, so do not even see what you say about alternator trying to save fuel - charging looks as turn on close to max what is safe for battery (14.8V).
After first rechargingaround 4 weeks ago, I got S&S working for a week, then it disapperared for a few days, then appered on one day, then gone for good again. I checked and status reported was 63%. Value didn't tell me anything, though I consulted with other guy on polish forum, who has 2016 Giulia Super, and has S&S operating all the time. He said that his status in the time I asked him for check was 83%. I did a few longer rides (like 50-100km over christmass time), and my S&S started to work. Charge status reported was 67%. Then I did another long journey, expected to bump up the SoC value, but nope, 66% reported. I started thinking something is wrong with either the battery or IBS poorely estimating battery SoC %s. So I connected charger, 10h of charging, current starting from 2.5A, soon dropped down to around 1A, then later to say ~0.5A, all was setup to avoid exceeding 14.8V. I managed to bump to 72%, but after 1 minute it shown 71%. After around 1-2h I measured battery voltage (nearly unloaded, I left boot open, closed car, waited some time, so only alarm and central locking was on, measured voltage at battery, was showing 12.73V, so probably pretty good). Next day I did 20km ride in the town (S&S works again, hurray), went back home, see SoC 70%.
My current suspicion is that IBS design is piece of rubbish, that probably it estimates in very crude way SoC, and so some cars have problems as they are below thresholds set to activate S&S (nonethelesss probably are still at pretty good SoC level), and at more difficult periods (winter, short distance rides only), they even go below (according to IBS) threshold set to deactivate things like rear screen heating.
I will be taling to my dealer next week, I'm planning to propose them replace of IBS to see if that chages anything other option is battery I guess, though mine seems quite good I think (I have also 2 tests done on my by dealer in Nov, one after first 3h charging, and other straight after some addtional charging they run (attached pictures below)