Looking at http://cgit.drupalcode.org/commerce_paymill/tree/src/PluginForm/Paymill/... it seems that the name attributes of form elements are never unset, causing the data to still be sent to the server, which is the same bug that Commerce Stripe had before Stripe Elements was introduced.

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bojanz created an issue. See original summary.

bojanz’s picture

bojanz CreditAttribution: bojanz at Centarro commented
Status: Active » Closed (works as designed)

Nevermind, there's a #process callback that removes the name attributes, vasike pointed it out.