Short answer - your preheader text is too short.
When your preheader text is too short, text from the email will be visual. Add more text or space.
Unfortunately it doesn't help to add some space with your space bar or hit the return key at the end of the preheader text to add space between your preheader text and email content.
The most common little hack (adding ͏ ‌ - zero-width non-joiners and non-breaking spaces) that was used isn't longer supported by Apple mail.
Apple mail needs ­ also known as soft hyphen. The only problem is that some outlook versions will show the soft hyphen instead of a blank space. Therefore, we recommend to add  ͏ (that is supported by most other email clients) first and ­ at the end.
As the old fix, the combination needs to be added several times - how many depends on the length of your text
 ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏  ͏ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­
After you added the code to the preheader and leave the editor but come back, the code won't be visual but the spaces are still there.
Different email clients have a different length of preview text and depending on the width of your email client window, more or less text is visual. Therefore, you might not always be sure that no email content appears in the preview text.