The content of flexible fields seems to disappear when presented inside a tab field. Removing tabs has made the content reappear.
1. Content can be added to the flexible field when it is in a tab. Content can also be saved, and will show on the website.
2. Returning to the admin area after a save, the flexible field area will appear empty. It is bugged though, as it will not have the usual appearance of an empty flexible field (Text reading click below to add content button). Saving the page will empty the flexible field (content will not show on the webpage anymore and text reading click below to add content will be back).
3. Removing the tab fields from the ACF page and returning to the page in the admin area will make the content reappear, provided you did not save the page (as described in step 2).