This problem stems from a gap in Lightning Experience surrounding Contact creation, which is exposed through the use of Duplicate Management. Standard Contact Matching Rule evaluates the 'Mailing Address' field whenever the lookup to Accounts is populated. The system uses this information to warn users of potential duplicates prior to Saving the record. This works without issue in Aloha (Classic). When a user populates the Account lookup in Lightning (on new Contact creation), the 'Mailing Address' is not pulled down from the Account level. That lack of that mailing address prevents the system from identifying the potential duplicate. Even custom Matching Rules that try to match on 'Contact.Account' are affected.
Workarounds available for Contact Matching Rule failures in Lightning
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