HTML button missing on issue page in OJS3

Our journal, REGION, offers papers in PDF and HTML format. After creating an external HTML galley, the HTML-button shows up on the article’s landing page (see e.g., Integrated local development in Mediterranean marginal territories: The case study of Casentino (Italy), Algarve (Portugal) and Corse (France) | REGION), but NOT on the corresponding issue page (REGION). We are using OJS3.
Is this a bug or a feature? Is this a problem with my settings?

Any help will be appreciated.