Why forcing mandatory fields when updating module entry?

Something that's causing me trouble and is resulting in not being able to optimize the use of 10 allowed Workflow rules when creating Flows for Books: forcing mandatory fields to be updated when updating module entry, Invoice in particular.

My case: we added a custom fee in "Contacts" in Books named "Reseller". Then, we created the same custom field on Invoices. The Flow I'm trying to build would be to update that field on Invoice when an Invoice is either created or edited by fetching it from Contact.

But the problem is I'm not able to just update that field, I am forced to update a bunch of "required" fields on existing Invoice for no good reason, including "Line Item", and the problem is, it doesn't work properly as our Invoices typically have more than one line item.

My question is, why would we be required to update required fields when updating an existing Invoice? If the Invoice is existing, it's very safe to assume that those fields are not empty, so why forcing to set new values?