I don't know but it is not the first time it has happened to me.

I ended up "clear formats" to get it to stop. I suspect it might have something to do with Excel trying to format a cell based on the formatting of cells in the rows above. Prior to changing the cell back to "General" I had highlighted a range and changed it to "Text" then highlighted a cell and copied and pasted to several cells. Later I tried to enter formulas in a couple cells and changed those back to "General" but as I stated before I had to press F2 to get them to calc and then if I pressed F2 again they reverted to "Text".

At the moment though I can't reproduce the problem either.

Thanks for your attempts to reproduce/solve the issue.