Why am I getting inconsistent results with checkout.line_items in Liquid script?

[Scooch]

I am trying to collect data from the order status page via Liquid script but I am getting inconsistent results when accessing checkout.line_items. 

I added the code below to the order status page additional scripts section, and for some orders I get the correct amount of items, but for some I get 0. I can't find a pattern, but it's consistent between orders (once 0 always 0, once correct always correct). For some orders this prop exists, for some it doesn't.

Obviously all orders have items.

Can anyone shed some light please?

<script>

  console.log('checkout.line_items.size', {{checkout.line_items | size}});

</script>

[Scooch]

To complement, the whole checkout object seems to not be present. This line of code prints nothing:

console.log('checkout.id', {{checkout.id}});