Cart/add.js response intermittently includes empty items array

Short description of issue

cart/add.js response intermittently includes empty items array

Reproduction steps

Difficult to reliably replicate, as it seems to be an intermittent issue.

Additional info

When it happens, the response looks like this:

{items: []}

Instead of the array including all the added line items.

Not sure if it matters but this is on a Plus store, testing specifically on legacy account pages (/account and /account/orders/xxxxxxxxxxxxxxxxxxxxxx). The site also does have a custom Functions discount checkout extension installed on it that runs on cart.

What type of topic is this

Bug report