Excel formula automatically rewriting itself??

Hi all, this is a really odd problem to run into and I am unsure how to proceed from here. I am currently entering data and I have Excel performing an =SUM(B5:AC5) for my data that is being entered. Because the data entry can go pretty fast sometimes, I would keep entering data past AC5 before I realize what happened. When I look at my SUM column, Excel has flagged my =SUM(B5:AC5) formula with the error below, which is fine and whatever, but it is automatically changing my =SUM(B5:AC5) formula to =SUM(B5:AF5) or something of similar depending on what column I stopped at. Just for reference, data from AD5 to AO5 is being summed up in another column with SUM=(AD5:AO5).

Formula error being flagged by Excel

I have no idea why it is doing this and I tried to ignore error, but it still automatically rewriting my formula anyways! This worksheet and formula worked smoothly 4 months ago so I don't know what is going on.