I tried uploading new images through the website and then the watermark is displayed.
But, when I try uploading new images via the API tool, the watermark isn't displayed.
I've just test it and the API does apply the watermark on 2.3, there is nothing need to change in the API because the watermark implementation is on the core of class.upload.php
Please try to test it with the same image, watermarks doesn't apply on animated images.