Try to create header using header builder https://youtu.be/RbdKjQrFnO4
Let me know if search by SKU works fine when header is created with header builder and when you use search element of the header builder. Keep in mind that the default header structures will be removed soon and the header builder will the main option to build header.
Or provide us with temporary Dashboard access to your site if you need help with header creation.
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