Excel Autosum putting in cell reference

One of my users is trying to use the AutoSum feature in Excel to add two cells horizontally.  We are trying to add cell C4 and D4 together with the result in cell E4.  When we hit autosum, you can see the formula in E4 for less then a second then it puts a cell reference in there ('2018 P&L By Month'T13).  Now for the strange part, when we open another spread sheet for testing, and do the autosum in A3 to do and addition of A1 to A2, you see the formula for a second then it inserts the same reference point as on the last sheet.  I did go to Calculation Options and confirmed Automatic is selected.  I've searched this on the web, but don't see anything out there.  I did have a ticket open with MS on this and we "Band-Aid" it by switching to manual in the Calculation Options in the affected sheet, which works.  But doesn't take care of the issue since Automatic Calculations are needed for the size of this sheet.  Per the tech that was working with me, he told me "The excel sheet on which we worked has multiple formula’s and it saved in the excel cache therefore we had to switch Calculation from Automatic to Manual."  Is there away to find and get rid of the cause of this?

Thank you

Dave