The integration of the Nextcloud client with the OOreader reader on IoS devices (iphone / ipad) fails. When you try to open a Libreoffice document (ODT, ODS) through the NextCloud client with OOreader it gives an error that the document has zero size and does not open it.
If you are sure this is a bug and not a config issue, feel free to log a bug at https://github.com/nextcloud/ios/issues or https://github.com/nextcloud/server/issues (but not both).
Regards