It could be a couple of factors. If it's moist/wet and it's still wilting, that could be a sign that the roots have died (rotted; they can no longer take up water so the leaves suffer).

It's also possible the soil it's in is hydrophobic, meaning when you water, it doesn't soak all the way through (a lot of peat based potting mixes do this. When they dry enough, they harden, and it's difficult to re-wet them).

When you say brightest spot possible, do you know what direction the window faces? West or South would be too intense IMO for a Philodendron.

Planto

does the pot drain????

you are in italy .... is air conditioning an issue... no cold breezes on it, are there ????

it is stressed... for now.. stop feeding it ... its not hungry ....

ken

Agreed with the above inputs. "The soil was just moist all the time."

Overwatering isn't about the amount of water given to a plant, but about whether or not there is oxygen in the soil. Roots need oxygen and moisture at the same time to function. If it would have been called underdrying from the get-go, it wouldn't be so confusing.

In a plastic or glazed pot, roots have no access to oxygen unless it is in the soil. Most potting soil is too dense, the tiny particles lodge together and eliminate the air. To compensate, it's usually necessary to let the soil dry fairly thoroughly. This exposes plants to wild swings of moisture/oxygen levels, which is not ideal. Changing to a more chunky/porous/airy/fluffy soil can help a lot of people (those who tend to think plants are always thirsty, soil must be kept moist.)

Once the roots reach the bottom of the pot and start circling, they're especially sensitive to sitting in water/suffocating, and from being lost to drying and shriveling. If that happens, then there's a layer of dead roots at the bottom of the pot. If there's then period of moisture, those dead roots will start anaerobically decomposing. Such close proximity to that can make the still-alive roots ill.

I'm not totally sure if I have the "why" of the above paragraph scientifically correct, but I've seen this happen, so I try to repot plants before a serious 'pancake' of roots gets going at the bottom of pots. I'd prefer to not investigate this more ;)