Hi there @gdcf
Apologies for the response delay here.
I installed that Simple Masonry Layout plugin on my test site to see if I could replicate the issue there.
Indeed, once I set it up on a page, I could see the same issue as in your screenshots.
However, when I deactivated all other plugins besides Smush and Simple Masonry Layout, the post grid displayed as expected and I could not replicate.
So I concluded the issue must be a conflict with some other active plugin. On my test site, that conflict turned out to be our Hustle plugin. As soon as I activated that one along with the 2 others, the issue re-appeared.
So I would suggest temporarily deactivating all other plugins on your site, except of course Smush & Simple Masonry Layout, then check for the issue.
If things display as they should, and I suspect they will, begin reactivating plugins one by one until the issue re-appears, and you’ll have found the conflict on your site and we can work from there.
Cheers!
Patrick