theme breaks gallery

I’m using the 320 wp-bootstrap theme, but this also applies to my custom theme. Inserting the wordpress gallery I end up with just a single image showing up on the page. This is a local build so I can’t link to it. When I inspect the element there is just a single
<li> in the <ul>.
I don’t want to hack into core, so there must be something missing from the theme.

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