The error in question may occur if StrixNetwork.instance is used in OnDestroy when finishing a play in the editor during development.
Unity destroys GameObjects in the scene when finishing a play, and it reports the error in question if there were undestroyed GameObjects remained after the process. In general, it is often caused by instantiating a new GameObject when OnDestroy is being processed.
The entire error message will be:
Some objects were not cleaned up when closing the scene. (Did you spawn new GameObjects from OnDestroy?) The following scene GameObjects were found:
StrixNetworkManager is a GameObject that Strix Unity SDK uses internally. When a C# script refers to a singleton instance of StrixNetwork (StrixNetwork.instance), StrixNetworkManager is instantiated. It is usually destroyed at the end of a play together with the singleton instance of StrixNetwork.
When StrixNetworkManager is listed at the end of this error message, it is likely that the error in question is caused by an access to StrixNetwork.instance from inside a callback such as OnDestroy() that is called at the end of the play. You can avoid the error either by not referring to StrixNetwork in OnDestroy() or by performing any STRIX-related post-processing only after checking StrixNetwork.isInitialized returns true. (It returns true when the singleton instance of StrixNetwork is present. It returns false after it has been destroyed).