Archive/category pages display two blog posts side by side?

Since installing Shapely, the archive pages for specific categories have started displaying oddly. The first post shows up normally. After the first posts, all subsequent posts appear side by side in two columns. It is almost impossible to read. (See here for an example: https://www.annasaxton.com/category/reading/). I should not have any plug-ins that should be making this change. Any suggestions for fixing this? Thank you!

Resolved

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Thanks!