I cannot reproduce what you are describing.
The "@" is a very old artifact that was used in other spreadsheet programs like Lotus 1-2-3 and I think Excel may have allowed it for compatibility at some point. But don't use it. It has another meaning in new versions of Excel and will cause problems.
That's why I asked for keystroke-by-keystroke. Above you said you start typing =(
but do you mean =SUM(
?
Even if I deliberately enter a formula that is missing a paren, I don't get that message. I get a message that there is a "typo" and it offers to fix it for me. If Excel 2013 has behavior that is a lot different than 365 this will be hard for me to diagnose much further.