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 the pre-header text to add space between your preheader text and email content. But there is a little hack that is commonly used in the email world.
a couple of times (some rows) at the end of the preheader text and the remaining text space will be filled.
It's a good idea to add a space with your space bar before you add all the zero-width non-joiners (͏ and ‌) and non-breaking spaces ( )) eg:
text text 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.