Hi @Ben Knowles ,
Thank you for posting in this community. Note: SharePoint will reindex all of the content in this List during the next scheduled crawl. It takes some time to reindex list, between 15 minutes and 24 hours there is a difference. We'd also recommend you wait for 24 hours to check after re-index.
If that doesn't work, You can go to SharePoint site > site settings > search schema to check if the property is mapped with correct crawled properties. For your reference: Manage the search schema in SharePoint - SharePoint in Microsoft 365 | Microsoft Learn
You can try to search by prefixing your search with Managed Properties. Follow the instructions in the diagram below:
Hope the information can help you. And if there have any unclear or misunderstanding, please feel free to post back and we’ll continue to help you all the time!
Your understanding and patience will be highly appreciated! Hope you have a good day and keep safe!
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.