Hello @Mirella,
Welcome to Microsoft Q&A forum. Based on the information you provided, it appears that the issue occurs when enabling the “Make this section collapsible” feature on your SharePoint page.
To replicate your scenario, I created a new section using both Flexible and One Column layouts as shown in the screenshot below.
After enabling “Make this section collapsible” and adding a heading, the behavior matched what you described in your thread.
However, once the page was published, the heading automatically displayed on a single line.
Could you please try publishing or republishing your page to check if the heading adjusts automatically based on the device you are using?
Thank you for your cooperation, and please let us know the outcome so we can assist further if needed.
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