This is a follow-up from SERVER-125857. Example:

{ x: 5, arr: {$elemMatch: {a: {$ne: 1}, $or: [{b: {$lt: 2}}, {b: {$gt: 3}}]}},} 

The negation and the $or share the same $elemMatch context, so the or-pushdown would be correct, but it is currently not Implemented when the $elemMatch is not a root expression.

The limitation applies only to negation predicates, that are tracked specially in plan enumerator. The pushdown of other predicates is not affected by the presence of the sibling {x:5}