Hello @JustinZ
I cannot replicate the problem you have described.
If I use the GUI to put a particular field on the Create screen of a particular Issue type in a particular Project, I can see it on the screen when I try to create one of those Issues. If I then use the Get create field metadata for a project and issue type id endpoint to lookup the fields on the Create screen of that Issue type in that Project, that field is returned in the response.
If I then remove that field from that Issue type’s Create screen (using either the GUI or the REST API) then I re-query that same Issue type via the Get create field metadata for a project and issue type id endpoint, exactly the same as before, that field is not returned in the response.
I don’t experience the ‘many hour’ lag in the data updating that you have described. Also, I’ve not heard anyone else report such a lag / delay in the response data of that endpoint on this forum or the public Community Forum.
Double check your work and, if you can prove there is a definite lag in the data being returned by that endpoint for your Jira Cloud instance, I suggest you open a support case with Atlassian.