This is a tracking bug for a number of memory-related issues that block parsoid from fully replacing the legacy parser (T55784: [EPIC] Use Parsoid HTML for all page views).
Some of these are PHP environment configurations which differ between Parsoid and the core mediawiki cluster.
Others address the resource limits put on wikitext, which would cause pages which render in the legacy parser not to render in Parsoid, or vice versa.