I've got 3 default image sizes set up in "Setting > Media Settings > Image Sizes"
However, when I use anything but "thumbnail" as the image size parameter in the_post_thumbnail(), it returns the full size image.
Am I doing something wrong or could there be an active filter making this happen?
For example, this should return the medium 300x300px max size image:
And this should return the "large" 1024x1024px max: