Hi,

You have to loop on line qunatity field and modify the value field to have the unit.price but in company currency and since there no filed for signed unit price
you will divided the self.price_subtotal_signed field by line qunatity to get unit price signed.

So you will ovveride asset_create as below and you donot need to call the super:

@api.one
def asset_create(self):
    for x in range(self.quantity):
        if self.asset_category_id:
            vals = {
                'name': self.name,
                'code': self.invoice_id.number or False,
                'category_id': self.asset_category_id.id,
                'value': self.price_subtotal_signed / self.quantity,
                'partner_id': self.invoice_id.partner_id.id,
                'company_id': self.invoice_id.company_id.id,
                'currency_id': self.invoice_id.company_currency_id.id,
                'date': self.invoice_id.date_invoice,
                'invoice_id': self.invoice_id.id,
            }
            changed_vals = self.env['account.asset.asset'].onchange_category_id_values(vals['category_id'])
            vals.update(changed_vals['value'])
            asset = self.env['account.asset.asset'].create(vals)
            if self.asset_category_id.open_asset:
                asset.validate()
    return True