on this page https://help.shopify.com/manual/apps/apps-by-shopify/script-editor/shopify-scripts#split-example
the split example has the line
cart.line_items << new_line_item
but that throws an error in Script Editor saying "instructions quota exceeded". Without it, my split line item doesn't appear in the cart...
Could you provide the complete code you are using? Judging by the error message you are trying to run too many things in a single script.