Why does the timeline endpoint return different results on 2 times try with the same user id?

Sn0w · 2022-04-16T09:44:49.902Z

I try with the postman. With the same user id and no optional field:

First time: I get the empty result, only have meta with result_count is 0
Second time: I don’t change anything, just press the send button and now I got the result with 10 tweets, having next_token value.

The question is why the API returns an empty result, and I have no way to check whether the user tweet list is ended or not.