Issue with #REF showing for calculated cells when Excel is in Protected Mode

I have a workbook with multiple sheets - data is entered on the first sheet as calculations happen over the course of several sheets, resulting in final values in the 'Output' sheet.

When the file is emailed and then opened, it opens in protected mode (as expected), but the Output tab is showing #N/A for several of the calculated cells (but is showing the expected results in others).

This is an update to a previous version of this file where this was not happening (all calculated cells showed their values when in protected mode).

There are no links to any other workbooks in this file.

Anyone any ideas here?