What was going on
With "Enable Deep Image Optimization" on in the Fastly Magento2 extension, product list images rendered at the wrong size / were not resized properly. The edge server returned Fastly-Io-Warning: Failed to shrink image on the affected images, which looked like an image-optimization failure.
What fixed it
The "Failed to shrink image" warning was a red herring: the actual cause was a missing canvas query argument on the image URLs, so the image optimizer had no target dimensions to shrink to. Adding the canvas argument (e.g. &canvas=262:262) produced correctly sized images, and the reporter confirmed that was the issue. Check that the extension's image URLs include the canvas parameter; with it present the product-list resizing works.