While addressing T332931: Read views HTML and Edit views HTML for a page should come from the same ParserCache entry, we broke visual editing for pages with multiple MCR slots (see T351026: VisualEditor adding nonsense code to file pages ).
To address T351026, we are currently deploying a hack where we always return the 'main' slot output for Parsoid HTML requests. This will however now break Parsoid read view for these pages since we will only be returning the main slot output.
We should figure out a strategy (internal and external API fixes) that lets editing and reading clients get the appropriate output for pages that have multiple MCR slots filled up.