I don't know the answer...But a couple of thoughts...
I've never seen Kerria growing in a "pot" before, apart from 3litre/5litre pots in garden centres. Maybe the pot could be the problem ? All the ones I've seen are very vigorous, suckering shrubs, say 1.25m wide at least, by 1.8m high....And there's always a lot of dieback which needs pruning out to healthy wood, along with some of the old wood to let the new growth flourish...Maybe if the top growth dies back it's worth seeing if the roots are still live ? A bit of watering/feeding, and give them a chance to sprout new growth ?? They often do.
They do fantastically well in Northern England in moist, cool, shady, north-facing conditions, with space to grow. I'm not familiar with Rome but a pot on a balcony in hot conditions might not be ideal...? Is it a shrub commonly seen in the Mediterranean ? ....But, fair enough, as you say it's seen in parks around the city. Maybe it's the pot...
...We have the same problem attempting to grow French Lavender in Newcastle....It generally just turns grey !!