Hi – I am having the same problem, i.e. cart is not emptied after purchase has (successfully) completed if MailChimp plugin is activated, while cart is emptied as MailChimp plugin is not activated.
Versions:
Wordpress: 4.9.4
WooCommerce: 3.3.3
MailChimp for WooCommerce: 2.1.4
@mcsupport: Is there (already) a fix or known work-around for this issue?
Thanks, Pim