Mobile-first indexing
Google using the mobile version of a page's content for indexing and ranking.
Google moved to mobile-first indexing over several years and now applies it to essentially all sites. Content, structured data and links missing from the mobile version are, in effect, missing from Google.
A responsive design that serves the same HTML to every device avoids the problem entirely.
Crawling & Indexing timeline · Latest crawling & indexing news · What Google said