TMP_Input Field Destroys text

my guess would be that this is an issue of your UI Layout or the text component settings. There is a "horizontal Truncate" option that you could try to remove. Aso try to raise the width of your text element.

For those who are coming across the same problem, I still have not found why this happens, but my solution was to create an enumerator where I set the text in the next frame instead of the same frame as below:

 IEnumerator CreatePostHelper(...) {
         // Instantiate caption GameObject
         GameObject postPanel = GameObject.Instantiate(PostPanelPrefab, PostPanelContent);
          ...

         yield return new WaitForEndOfFrame();

         caption.GetComponent<TMP_InputField>().text = postData.caption;
         yield return null;
 }