multi=true updates can modify the shard key of orphan documents and cause them to become owned

// If the shard key fields remain unchanged by this update or if this document is an orphan and

// so does not belong to this shard, we can skip the rest of the checks.

if ((newShardKey.woCompare(oldShardKey) == 0) || !metadata->keyBelongsToMe(oldShardKey)) {

    return false;

}

...

// We do not allow updates to the shard key when 'multi' is true.

uassert(ErrorCodes::InvalidOptions,

        "Multi-update operations are not allowed when updating the shard key field.",

        !_params.request->isMulti());