Attribute value gets cleared after being set by custom script

I'm getting this weird behaviour when updating an attribute after another one has been changed. The value in new_manual_invoice_number is somehow cleared:

function handleInvoiceNumber(context) {

        debugger;

        if (Xrm.Page.getAttribute('invoicenumber').getValue() !== undefined) {
            if (Xrm.Page.getAttribute('new_manual_invoice_number').getValue() !== undefined) {
                var manualNumber = Xrm.Page.getAttribute('invoicenumber').getValue();
                manualNumber = manualNumber.split('-')[0] + '-' + manualNumber.split('-')[1];
                Xrm.Page.getAttribute('new_manual_invoice_number').setValue(manualNumber);
            }
        }

};

This is running when invoicenumber is changed (it's a read only field and changes when the invoice is saved).

This works fine, I can see the value appearing in the form, but somehow, something clears it and I'm getting -- in the text box. It's not any of my other code, there's only thing function refering to new_manual_invoice_number.

I've tried running this code from on a save handler, form load, etc. same behaviour.

I've put a breakpoint in the form load's event too to see if it was not reloading due to auto-save or something, doesn't seem to be the issue.

Any ideas ? Quite frustating...

PS: Using Microsoft CRM 2016.

EDIT: I've also added an on change event on the attribute new_manual_invoice_number and it's never triggering :O