How to Stop Excel Converting Formula to Value Automatically

Cross-posted at https://answers.microsoft.com/en-us/msoffice/forum/all/prevent-excel-changing-excel-formulas-into-values/bdef3897-915c-4fba-beae-a41b77297090

I have a large and complex spreadsheet, but a simple problem that I cannot resolve.

In one sheet, my formulas are being converted to values by Excel automatically. I do not want this to happen, I need them to stay as formulas and recalculate every time the data is updated.

So for example, I enter into cell A1 the formula = B1 + C1
The value are 2 and 3 in cells B1 and C1 respectively.
When I press enter, it will correctly calculate this as 6.
However, it will also auto-convert the cell A1 to the value 6, and the formula is lost.

If I then update the contents of Cell B1 to 4, the formula no longer exists and so A1 is not updated.

What is going on? Is it a bug? I would be grateful if anyone has any ideas.

Thank you!