Server Version#: 1.31.0.6654 (Synology DSM 7)
Player Version#: 4.87.2 (Plex Web)
If you disable collections showing in the Library (Advanced > Collections set to ‘Disabled’) and have no collections in this library, when you create the first collection, the Collections tab does not display.
You have to change the setting back to show, then disable again.
based on experience it might take the web app to reload for the collections tab to show up after creating your first collection. This should be unrelated to the Collections option on how they’re being displayed inside the main library tab.