The issue doesn't seem to be with the app indicator extension - or at least not solely. When running the Nextcloud client as AppImage, be it the latest 3.17 or 3.16.6 from [1], the indicator is showing up properly. Though I can't tell what exactly the difference is between the app image and how Debian compiled it.
Best regards Richard [1]: https://github.com/nextcloud-releases/desktop/releases/
