Problem with makeing text appear on screen

I have added functionality to speak with NPCs and I wanted the text to be animated like the typewriter animation in PowerPoint (The text appears letter by letter). I have been able to do so but sometimes the text dissappears for a frame and then reappears.

The entire class is very long so I have included the relavent function. I achieve the animated effect by creating a substring of the original text which increases in length with time so that more of the text is displayed.

     void DisplaySpeech() {
         
         GUI.BeginGroup(new Rect(Screen.width*0.05f, Screen.height*0.7f, Screen.width*0.9f, Screen.height*0.25f));
         
         GUI.Box(new Rect(0,0,Screen.width*0.9f, Screen.height*0.25f),""); //Draws background for text
         
         GUILayout.Label("Text", titleGuiStyle); //A placeholder title will modify later
         
         string sub = interactions.interactions[interactionSelector].text[textSelector].Substring(0,Mathf.RoundToInt(charSelector));
         //The interactions.interactions[interactionSelector].text[textSelector] variable contains a list of strings which are displayed on after another
         //To achieve an animated effect the string is cut into a substring which starts at the beginning for a set number of characters which increase over time
         //The string is then applied to a Label
         GUILayout.Label(sub);
         
         GUI.EndGroup();
         charSelector += Time.deltaTime*textSpeed; //This variable increases with time and so allows for more the string to be copied to the Substring
         
         if(charSelector >= interactions.interactions[interactionSelector].text[textSelector].Length) { //If the charSelector becomes too large for the string
             charSelector = interactions.interactions[interactionSelector].text[textSelector].Length; //Set charSelector to the size of the string
             GUI.Label(new Rect(0,0,Screen.width*0.9f, Screen.height*0.25f), "Continue", titleGuiStyle); //Tells the player that the text has been displayed
             if(Input.GetButtonDown("Interact")) { //If the player continues
                 textSelector++; //Increasing the textSelector allows for the next string to be displayed
                 charSelector = 1; //Resets the charSelector
             }
             if(textSelector >= interactions.interactions[interactionSelector].text.Count) { //If the textSelector is greater than the number of strings
                 Reset(); //Reset all relevant values to this function
                 currentMode = modes.SETFLAGS; //Sets flags relavent to this interaction
             }
         }        
     }

To help people understand the interactions variable is a ScriptableObject containing data about NPCs.

It also contains possible dialogues, interactions.interactions[interactionSelector].text is a list of strings for a dialogue.