Does it do it the first time you type into the cell or sometime afterward, like after you've deleted something from the cell and are typing/pasting something new back into it? Some commands, like Cut (cmd X), will remove formatting too.
You used the phrase "when I enter data into the cell". Are you typing into the cell or pasting into the cell? Pasting will override the format. Paste & Match Style does not.
It never does it the first time i type in data.
I think it's happening when I cut from the cell and enter something new in by typing. I didn't realize cells could lose their formatting like this.
I'm going to see if it only happens in these circumstances but I think this is the issue. Thanks for your replies.