Apologies for the very late response.
I did eventually manage to solve the problem. I went on the edit page section in the main wordpress admin area and turned off 'Lazy Load for Images' under the Cache options (I'm not sure if this is related to WP Rocket or is just a general Wordpress setting).
This has stopped The Grid from squashing up in the way that I described earlier. However, the side effect is that the page loads quite a bit slower than before as there are a lot of images on my home page and this setting, presumably, turns off image lazy loading for all of the images on the page.
Ideally, I would like Lazy Loading to be on for all images on this page except those images that are produced by The Grid. Do you know how I can achieve this?