After further investigation, it seems you are updating the metadata. My apologies.
My particular situation comes when a particular uploaded image just barely meets the image size requirements. Case in point:
I define a size "My_Img_Size" and give it a width and height of 600px each. I then upload a 600x400 image. The plugin does not generate an image (copy) that is that size. Because the width of 600 matches one of the defined max's, it just assumes 'full' is good enough.
As it is, to make do, I have to first run image_get_intermediate_size(), and if it doesn't find the right sized image (the full size), I then must run image_downsize(). If you created the custom size that happened to match full size, I could just run wp_get_attachment_metadata() and go home early.