Cost-based planner
What changes in Cost-based planner compared with MongoDB and what your application has to do.
This difference is of the Not guaranteed by MongoDB kind.
What MongoDB does
The planner picks a plan by trying candidates. A query without sort comes out in that plan's order.
What TeselaDB does
The planner picks by cost and may choose another index. explain, $planCacheStats and the order without sort change, never which documents are returned.
What your application has to do
If you depend on the order, ask for sort. Do not compare the output of explain with MongoDB's.