StrictModeError not thrown on findOneAndUpdate with unknown field

Background

Mongoose’s strict option controls how unknown fields are handled. When set to 'throw', attempting to set a field not defined in the schema should raise a StrictModeError. This behavior is well‑documented for document creation and doc.set() operations.

Unresolved Behavior

During findOneAndUpdate calls that use MongoDB update operators such as $set, the error is not consistently thrown even when strict: 'throw' is specified. The operation sometimes silently strips the unknown field, leading to data loss without an explicit error.

Goal

Determine the exact conditions under which StrictModeError is suppressed during findOneAndUpdate when strict: 'throw' is enabled, and whether this is a documented limitation or an unintended bug.

Questions

• Is the suppression of StrictModeError on findOneAndUpdate with unknown fields a documented feature or a known issue?
• What role does the runValidators flag play in this scenario, if any?
• Are there any configuration workarounds or recommended patterns to enforce strictness on update operators?