Confused about "available offline"

Nextcloud Android client version 3.5.1 on Samsung Galaxy A3 (Android 8.0.0).

If I understand correctly, files that are downloaded to the client will stay synced. IOW, when the document on the server changes, the client will automatically pick up the changes upon the next sync cycle.

The behaviour I see is

  • the downloaded document has a green ‘downloaded’ mark
  • changes are made on the server
  • the document now gets a red cross mark

It doesn’t get synced, even after a long time. Nextcloud is configured to not be put to sleep by power management. Are there any additional settings needed?