Hello Aaron,
I tried replicating the issue mentioned by you but was unable to do so at my end. Please refer to the attached sample wherein the FormulaTextBox has been attached to the Spread and saved to Excel. On loading the Excel, and putting the cursor in the cell showing the value of the applied formulas, the value is not lost. Please refer to the attached avi.
Also, let us know if we have missed anything. In case you still face any issue, you are requested to share your sample and the steps performed by you with us.
Regards,
Mohita