A similar question has been answered in 2015, but it was not resolved: Contribution email receipt with custom fields shows array in template
I'm having a similar issue. But instead of custom fields returning "array" to the recipient, it is returning field attributes. Since there are many custom fields on this form, the email is screenshot after screenshot long. This happens not only for anonymous users, but logged in users and email receipts sent to staff.
There are no errors in the logs. While inspecting, the only thing I see is a warning: Synchronous XMLHttpRequest on the main thread is deprecated because of its detrimental effects to the end user's experience. For more help, check https://xhr.spec.whatwg.org, which doesn't seem related.
Any help would be appreciated.