the_post_thumbnail(“size”) returns same image for all sizes except thumbnail

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:

echo the_post_thumbnail("medium");

And this should return the “large” 1024x1024px max:

echo the_post_thumbnail("large");