While investigating a failure, catalin.sumanaru@mongodb.com discovered that When pushing down a $lookup + $unwind in trySbeEngine mode, the QSN tree keeps an extra UnwindNode here that is not explicitly part of the QSN tree. This node does not get assigned a nodeId, which causes problems later on with plan caching.

This is only possible to hit in trySbeEngine mode, so it should not affect any users running in standard/supported configurations.