Oh, I found one of the bits that had been modified in mainScene instead of the JoinCreateCanvas prefab; Looks like the assignment of Hide on the join button OnSelect wasn't made to the prefab.

Assets/Prefabs/UI/JoinCreateCanvas.prefab

     m_TransformParent: {fileID: 1119140321553661053}
     m_Modifications:
     - target: {fileID: 433211913614645534, guid: c308ffc2a02e5ab4bbe70a8b2e8108c6, type: 3}
-      propertyPath: m_OnClick.m_PersistentCalls.m_Calls.Array.size
-      value: 2
-      objectReference: {fileID: 0}
-    - target: {fileID: 433211913614645534, guid: c308ffc2a02e5ab4bbe70a8b2e8108c6, type: 3}
       propertyPath: m_OnClick.m_PersistentCalls.m_Calls.Array.data[1].m_Mode
       value: 1
       objectReference: {fileID: 0}

Assets/Scenes/mainScene.unity

       propertyPath: m_AnchoredPosition.y
       value: 0
       objectReference: {fileID: 0}
-    - target: {fileID: 2918310285625255774, guid: f1d618bdc6f1813449d428126e640aa5, type: 3}
-      propertyPath: m_OnClick.m_PersistentCalls.m_Calls.Array.size
-      value: 1
-      objectReference: {fileID: 0}
     - target: {fileID: 2995192838028648417, guid: f1d618bdc6f1813449d428126e640aa5, type: 3}
       propertyPath: m_AnchorMax.y
       value: 0

Assets/Scripts/Game/GameManager.cs

         private RelayUtpSetup m_relaySetup;
         private RelayUtpClient m_relayClient;
 
-        //the Lobby API limits requests to 1.5 a second, and will return a 429 "Too Many Requests" error otherwise.
+        // The Lobby API rate limits query requests to one every 1.5s, and it will return a 429 "Too Many Requests" error otherwise.
         private const float k_lobbyAssignmentCoolingSeconds = 1.5f;
         private bool m_coolingDown;