Thanks – I’ve already updated everything to the latest version, and tried various settings before reporting this. It still happens on the parent theme and with all plugins disabled except Generate Blog. So I hope you can reproduce it.
That fixes it for my specific settings, with a Small grid. But when I test setting the Masonry Block Width to Medium instead, it pushes everything into a single column again (regardless of the Most Recent Width or Masonry Post Width).
I also have this problem – I create an aside post that I want to carry an Advertisement in amongst the small blog masonry tiles, setting it to Medium Size – opting out of the Masonry Global settings – and it breaks the site.
I managed to create one Medium tile that kind of works but only if I leave it where it is – If I change the date of publish of this tile to bring up to date so it’s seen near the top of the blog (advertising) it breaks the site. I’m forced to leave global masonry settings to small tiles.