I'm having the same issue, though I have started from a blank list. I have also copied the formula and adjusted from the 'Issues' template.
After saving the formula, when I return it has stripped a set of brackets out.
I enter and save =DATEDIF([Deadline],[Reqd By],"d")
When I go to look it says =DATEDIF(Deadline,[Reqd By],"d")
I swapped the columns around and it still took them off 'Deadline.'
But in fact this is the second try as I was working with a different column and created 'Deadline' fresh just to test.
All are set as Date type. It doesn't matter if I type the column or use the 'Add to formula' list.
Any ideas please?