I’m running a WP Glossary plugin by Codeat and I think it was causing the issue. I just disabled it on the page and its working fine now. I had another issue with the glossary affecting the excerpts incorrectly in WP Show Posts and asked the plugin developer to look into it. They said I had to disable the glossary on the page because I was using a page builder that was conflicting with the the glossary. Im not using a page builder for the record, so I assume he was just referring to conflict between WP Show Posts and Glossary. All good now though. Happy to just disable it for that one page.
