Hello.

Stripe Docs has the following:

Currently, only 'subscription' mode Sessions and 'payment' mode Sessions with post-purchase invoices enabled require a Customer.

'payment' mode is used for single time payments. Looks like we didn't hear about such problem before because 'post-purchase invoices enabled' by default and Customer is always created.

By the way, thank you for the research. We'll add this parameter to avoid such problems in the future.