If your preheader text is short, email clients fill the rest of the preview with text from the start of your email. To avoid that, make the preheader longer, or pad it with invisible characters.
Why spaces don't work
Email clients ignore extra spaces and line breaks at the end of the preheader, so adding them with the space bar or Enter doesn't help.
The old trick of adding zero-width non-joiners and non-breaking spaces (͏ ‌ ) no longer works in Apple Mail. Apple Mail needs soft hyphens (­) instead, but some Outlook versions show a soft hyphen as a visible character.
The fix
Add the pair  ͏, which most email clients support, followed by ­, after your preheader text. Repeat them many times; how many depends on the length of your text. Copy this example and paste it after your text in the Message preheader field on the email's Settings tab:
 ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­
When you leave the editor and come back, the code is no longer visible in the field, but the spacing is still there.
Results vary
Email clients show different lengths of preview text, and the width of the window also matters. You can't always be sure that no email content appears in the preview.