The preview function on WordPress isn’t working with custom fields. When previewing on WP it looks like the custom fields aren’t getting any values through. But when viewing on a published version of the site the custom field values are coming through.
I’ve tried looking at several other solutions to this issue such as this link here but they don’t solve the root problem as they are just replacing the preview post id with the published post id.
It also looked like a similar issue was known about in ACF 5.7.11 and the solution was to reinstall the plugin files. I tried this and it didn’t work.
Running WordPress 6.3.1, ACF 6.2.11 with the classic editor installed.