New Lightning Lead Conversion:

https://releasenotes.docs.salesforce.com/en-us/winter18/release-notes/rn_sales_features_core_leads_new_convert_ui.htm

We came across an issue that even though we select an account in the conversion process, if we select a contact in a different account, the opportunity created marks the account as the contact's account, not the contact we selected. We opened up a SF case (17564533).