Bear in mind I have already repaired the corruption using the exclamation tool. So, copying the cells to another sheet does as expected, that is, nothing unusual. The copied values are in the General format, just as they are on the original sheet.
You can change the format to Number in the second sheet and the Sum function still works as it should. That's quite different to how the original data was being interpreted - also in Number format. There, the original data sum was zero, because excel couldn't read those 'numbers'.
This data is hand-written, only the sum cell is generated.
Update: It's even possible to change the data to text, and excel can still sum the values. So where does this 'numbers as text' state come from?
Update: I have slightly reworded the post, to make it clear which values are the data that showed the issue - the list of values, which showed as 'numbers as text' and were non-computable, while the sum in the bottom right was incorrectly zero.