I have these high quality images that were rejected for "Quality Issues". Any advice? They're super sharp, hires, professional looking… This has happened with a bunch of my images, so I can’t quite figure it out. Thanks!

High resolution is probably making the weaknesses easier to see, not proving the files are high quality.

Copy space only helps after the subject looks commercially usable. In these sets, hard side light creates heavy shadows, the pink/turquoise backgrounds contaminate the food color, and several crops make the subject feel accidental. For food, buyers need appetizing texture and controlled highlights.

Take one frame and rebuild it: diffuse the light, neutralize the color cast, clean crumbs and edges, and leave intentional space around the food. Then judge it at thumbnail size first. If it does not look appetizing before zooming in, more pixels will not rescue it.

Your bread shots lack much aesthetic appeal or visual interest.

https://stock.adobe.com/search?k=loaves  11 million results

Food photography is an advanced sub-specialty that takes lots & lots of practice. 

Hope that helps.

This is what you’re competing with in Stock inventory. To get accepted, yours should meet or exceed current quality & visual standards.

https://stock.adobe.com/search?k=cake 11.5 million results

https://stock.adobe.com/search?k=birthday+cake

https://stock.adobe.com/search?k=wedding+cake

I think the composition could be improved. There is a problem as well with the lighting - it’s causing harsh shadows; the background is also throwing the white balance off (in my view). So ‘high quality’ in this case just may be subjective. 

Lighting, colors, shadows ...

It’s really not possible to judge the quality of your images from these small thumbnails.

Then also submit the bread as pngs. In any event, the third image of the bread is cropped too tightly.

Quality issues go beyond resolution, sharpness, etc. Here is your competition for homemade bread, for example:
https://stock.adobe.com/search?k=homemade+bread&search_type=usertyped

Your cake images may be better suited for pngs with transparency.