I’ve encountered this error also, it presents in Google Webmaster tools, in the browser if viewing the XML directly, and in the XML validator if tested there https://www.xmlvalidation.com/
XML Parsing Error: not well-formed
Location: https://www.domain.com/sitemap.xml
Line Number 102, Column 68: <loc>https://www.domain.com/?post_type=wpcf7_contact_form&p=1965</loc>
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