This is a feature. The @ sign is a wildcard, which shows the currently selected glyph. This is very useful because you can build a small set of preview strings to check any glyph in particular contexts, e.g. nn@nn and oo@oo for basic spacing tests.

If you need to display the @ sign in the preview window, use the convention /at . The slash is an escape character in preview strings after which you can identify a glyph by its name.

You can also use the convention \XXXX in which the variable is a Unicode character hex value.

That's not a problem, that's a feature. That's what the @
character does in the preview panel. If you want to see the
@ character, and not what's in the glyph window, type "/at".
You can do this for any character that can't easily be typed
or can't be typed at all. Just type a slash followed by the
name of the character. You may need to type a space after
depending on what follows.

Ahh. That makes sense. Thank you both!