It's a little weird.
I have checked the option for Accounting Preferences > Approval Routing > Journal Entries
It seems to be the culprit.
1. Manually, in the UI, the form has the field, so it takes into account your selection and the JE is created as approved.
2. In the SuiteScript, if no value is passed, defaults to Pending Approval.
3. Using CSV import the standard tool doesn't include the Approval Status as a field. So, it takes the default: Pending Approval.
4. There is ANOTHER CSV import tool for JE only: Transactions > Financial > Make Journal Entry > Import. Here you can set the Approval Status.
So, in my use case, I need to create a WF that sets a default value for JE as Approved, in order to affect both CSV imports and SuiteScript. If some conditions are met (amounts, etc.) I set it as Pending Approval and set the approvers.