i wonder if it is the apparent leaf that is the problem . i think your 3 year old got a leaf of easily because it was already ill . i wonder was it the one in the soil rather than the pot that has died ? if im right its the one in the boarder and the soil is dryer or wetter than the potted plants . thats my guess realy . there normaly quite tough plants . it has been hot and it is quite shocking being moved. it mite not have been treated very well before you got it . the point being is a healthy fatsia can easily handle losing a leaf . ive seen one get quite butcherd almost to the floar and come back . i have a fatsia and make concrete sculptures . when i make these sculptures i dont like wasting the left over cement so i cut leaves of my fatsia for a mould . i would investigate past the fallen leaf myself .

NP is quite right fatsias well able to cope with losing leaves in fact as they grow the bottom tier of leaves fade brown off and drop naturally.

I used to live in a house with a large victorian conservatory and the floor was covered in discarded leaves the fatsia had grown into a tree and had to saw it to prune it thats how robust they eventually become ..ps dont panic it takes many decades to reach that size .

lol bonkers

my fatsia is about 7`-8` tall and ive had it about 10 years from a little plant . i love it .

If its small enough to handle easily, turn it out of its pot, check the compost for pests - larvae, beetles, anything like that, even ants nest. While you're at it, check the dryness or not of the rootball.