Under a very specific set of circumstances, starting a formula with a plus causes other formula to change to R1C1 references (and don’t work because they are typed in like text).
So if you are old school and start formulas with a +, and you’ve just learnt to group sheets you could be experiencing this problem.
Below we have grouped 3 sheets and now want to enter the same formula (units x selling price) into the same cell on all 3 sheets. Note that we have started the formula with a + so it says +C1*C2
In this case when we click enter, on the sheet we are on, it works fine. But if you now look at the other sheets you will see that in place of a formula with the answer we have +R[-2]C*R[-1]C entered as text.
This seems to only happen when you use a multiplication. Although it might be obscure, it is fairly likely as an inexperienced user may just have learnt how to group sheets and multiplication is a fairly common requirement.
Main lesson- don’t start a formula with a +, ever. Start always with an = sign so Excel knows you mean to enter a formula.
For other reasons not to start with a +, see the Don’t start with a + in Excel post.