Editing single-line text or multi-line text fields in the Experience Editor might remove HTML tags after saving changes. The issue occurs if a field type value (single-line text or multi-line text) is not present in the Rendering.HtmlEncodedFieldTypes setting:
<setting name="Rendering.HtmlEncodedFieldTypes" value="text" />
To resolve the issue, consider one of the following options:
Alternatively, you can use the Rich Text field.