Thanks for the reply and the screenshots. I am still having the same issue.
I have cleared the cache on an iPhone 11 Pro and the same issue persists. There is no cache on the website as it is in development. In addition, I have tried viewing the same page and grid on an old iPhone 6 which has never visited the website before. I get the same behaviour as my original screenshot.
Additionally, I notice that on the iPhone 6, as soon as a cell has faded into view (correct as set in Effects), it then fades out again and disappears after a few seconds!
Could this be a grid bug which affects iPhones / Safari only?