I am not sure I understand well.

1 - You have insert API that is tested with postman and you receive acknowledgement and the inserted id. As understood by

Then, I assume that insertOne does work correctly.

2 - You have API code that reads back correctly from the same connection/database/collection as mentioned by

Then, I assume that insertOne does work correctly.

Since insertOne must be working correctly, then either you are not looking at the right place to see if the data is saved or the data disappear some how.

1 - What do you use to verify that the data is not saved?

2 - What version of mongod are you running and where it is running? An atlas cluster or local deployment?

3 - What else is done between the time you test the API with postman and the time you verify that the data is saved?

One possible reason why documents are removed automatically would be the presence of a TTL index. But your sample document have no timestamp or date field, so it could not be the case.

If you are not using Atlas, could you try to create an M0 cluster and use that for your tests.

Please read Formatting code and log snippets in posts so that the next time you publish code it is more readable.