Google Sheets - SORT formula working but data not visible

I'm using a Google sheet to collect answers from Google forms, compare some data and then create a chart.

The sheet actually works but the issue that I have is that while the =SORT formula works fine (I know it does because the chart takes its data from the results of the formula okay) the cells where it puts its results don't show on screen - it's as if it was white text on a white background.

Does anyone have any thoughts please on why this might occur?

I've put a anonymised version here : https://docs.google.com/spreadsheets...it?usp=sharing

The issues are on the red tab - any input is welcome!

Thanks in anticipation...

Ian

Without the ability to amend the sheet, I'd have to guess it's formatting that is hiding the contents.