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Hello Michael! I'm Jen, an independent advisor and a Microsoft user like you. I'd be happy to help you out with this issue.
Is this an issue with one particular file or all Excel files that uses formula calculations?
The calculation can change 'unexpectedly' if the first workbook opened in a session is set to manual then all subsequent workbooks in that session will be in manual when they are opened and this catches a lot of users out. Try to install the code mentioned in this thread below in your workbook(s) and in time as you use them they will all eventually get saved with auto calculation.
https://answers.microsoft.com/en-us/office/foru...
Another reason is that, it could be that the file become corrupted and prevents you from re-enabling automatic calculation. Try to copy the contents of the spreadsheet to a new one and save it.
I hope this information helps.
If you have any questions, please let me know and I'd be glad to assist you further.
Thank you and Best Regards!
Jen :)