In order for someone to help you with API Request questions, it is critical that you post here the API Request you are using.
Remember to replace your Key with [MY_KEY].

If I understand what your problem is, you are making two Requisitions and you are encountering inconsistency.
I think the best way for someone to help is if you put the two Requests here and indicate the ID of one of the movies where this inconsistency occurs.

The upcoming request endpoint. This provides a large list of 'upcoming' movies https://api.themoviedb.org/3/movie/upcoming?api_key={{api_key}}&language=en-US&region=US

However, if you choose a movie from the list. The movie endpoint for that specific movie usually has a different release date than the one the upcoming request listed: https://api.themoviedb.org/3/movie/488397?api_key={{api_key}}&language=en-US&region=US

As an example, I ran the first request in Postman, and one of the movies named "Wheely" (id: 488397) came up. The release date in the JSON data under the upcoming endpoint for the movie is listed as "release_date": "2019-03-22". If you then run the next request, which is for the movie "Wheely" specifically, the release date is listed as: "release_date": "2018-08-16"

This one has a very obvious discrepancy, and does not fall into the category of upcoming movie if it was released last year, and I am finding this in almost all the movies that are listed in the upcoming endpoint. I made sure to choose the same region for both endpoints in case the movie was released earlier in another country, which would account for this discrepancy, but it appears that does not help the issue.

I tried to enter the Release Information page of this movie example and ... No data. Anything.
https://www.themoviedb.org/movie/488397-wheely/edit?active_nav_item=release_information
Try it on and let me know if you found any data.
I already posted a message to the Moderators.
And I informed the link of our conversation.
Let's wait for news.

EDIT: And tell me what your OS is and your browser.
Mine is: Firefox + Windows 10 Pro

@She_Wolf The date returned in the "get details" method will be the primary release date. In your upcoming call, you're querying the U.S. data so that's what is showing. If you want to request the U.S. data on the movie details calls, you should make a call to the /release_dates method and grab the U.S. release date. There is no region filter for the get details method.

You can of course do that with append_to_response in a single HTTP call.

https://api.themoviedb.org/3/movie/488397?api_key=###&language=en-US&append_to_response=release_dates