Welcome to Shopify Community. My name is Olivia and I'm part of the Shopify team.
I appreciate you sharing your question with our viewers, especially as this is not expected behaviour. Before we move forward with various other troubleshooting steps, I'd like to rule out your browser as being the root cause here. Browsers do an excellent job of making our web experience smoother, and they do this by cacheing (or "remembering") our information and local data to help websites load quicker.
Sometimes browsers become overwhelmed with data, therefore, the data must be manually removed by clearing your cache and cookies. This should be done regularly, otherwise, websites can fail to load properly, or even prevent changes from saving such as ones made in the theme editor.
Please make sure to complete the following troubleshooting steps, and save the changes again:
Let me know if the issue persists at that point, and I'd love to take a closer look with you.