why is excel changing a formula in a locked and protected cell

I have quite a complex spreadsheet for use in my hobby of PBM - the spreadsheet includes an order form for players to submit instructions but with 1 player i am experiencing problems that I cannot fathom why it is happening

Upon emailing my spreadsheet, one of the cells in a hidden and protected part of the form keeps changing, the formula changes for no obvious reason creating an error as it is then trying to bring in/read data from an invalid cell. This only happens with this 1 person, everyone else who receives the exact same spreadsheet has no issue, the formula does not change and the spreadsheet works as intended - just not for this one person. the cell is hidden and protected, the sheet is also password protected - it should not be possible for this cell to change but it does.  Does anyone know why this might be happening? Both myself and the recipient of the spreadsheet run office 365 so there should be no issue with compatibility. as i say it is happening only to this one person, nobody else, and i do not understand why - looking for ideas on how to resolve this issue!!