I have two fields on SO, X on the body level and X1 at the line level. Both are being sourced from the Account field on the customer While the body field works fine, The sourcing is not working on existing lines on SO, If I add new lines on SO, it populates the value, but for existing lines on existing orders, it doesn’t work. What am I missing or doing wrong?

Does it work for existing orders for the body field? If so, is "Store value" checked on the body field?

Yes, it works fine for Body field, Store value is unchecked on both fields.

Are you sourcing it for the lines from the Customer (main) or Customer?

Customer, I cannot see any customer main in my list.

You may want to try different "Customer" fields that you find and see if anyone of them works.