Fatsia Japonica regeneration update

So I cut my mature, leggy fatsia japonica down to about 20 to 30 cm from ground level back in April/May.
The regeneration took a while to get started and then got into full flow around June time. However that little period recently where we had a 38°C day a huge downpour seem to correspond with some wilting of certainty the topmost leave.
I wonder if this is down to overwatering… Or some other factor? Pics below.
2 or 3 of weeks back:

Today:

2 or 3 of weeks back:

Today:

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They are tolerant of full sun if they have enough moisture - https://www.rhs.org.uk/plants/7147/fatsia-japonica/details