this works normally for me for smart previews synced from Classic. Sharp images in grid view on iOS Lightroom CC.
As an example, the first image is compressed and pixelated, selecting the middle photo (in any of the modules, including present) shows the same sized image, but at a much better sharpness. This is from an iPhone X with the newest version of iOS (though i get the same situation on an iPad). Its as if the gridview is using "preview" and selecting an image is using "smart preview". If there is a trick to fix this I'd love to know.
Ah that is a different grid view. I never used it but I just tried it and you are right. It uses a lower quality preview for that view that doesn't have enough resolution. Tried it on my iPhone 8. This seems to me to be a bug as you are right it looks pretty bad and as soon as you tap on the image it looks tack sharp. This also happens with files where CC has the full raw files so it is not related to the file originating from Classic. You should report on feedback.photoshop.com