It seems like you're encountering an issue where the `TextMeshProUGUI` text is not updating as expected. Let's explore some potential solutions to address this problem:

1. **Deferred Layout Rebuilding**:
   After setting the text of a `TextMeshProUGUI`, the `textBounds` and its size are not updated immediately. Unity uses deferred layout rebuilding, which means the update happens at the end of the current frame, just before rendering. You can try the following:
   - Set the text value.
   - Force a layout update using `LayoutRebuilder.ForceRebuildLayoutImmediate(myOutput.rectTransform)`.
   - This should ensure that the layout recalculations happen immediately after setting the text².

2. **Canvas Update Order**:
   Ensure that the canvas update order is correct. The canvas should update before any scripts that modify the UI elements. You can adjust the script execution order in the Unity Inspector to ensure that your UI-related scripts execute after the canvas updates.

3. **CanvasRenderer.SetMesh() or TMP_Text.UpdateGeometry()**:
   To manually update the text mesh geometry, you can use either of the following methods:
   - `CanvasRenderer.SetMesh(mesh)` (for `TextMeshProUGUI`): This method updates the mesh data for rendering.
   - `TMP_Text.UpdateGeometry(mesh, meshIndex)` (for both `TextMeshPro` and `TextMeshProUGUI`): This method updates the geometry of the text mesh.
   - You can find example scripts like `VertexJitter.cs` in the TMP Examples & Extras package that use the `UpdateGeometry` function⁴.

4. **Prefab Interaction**:
   Since you mentioned that the `TextMeshProUGUI` object is part of a prefab, ensure that the prefab instance in your scene is properly connected to the canvas. Sometimes changes made in the editor are not saved to the prefab itself. You can try using `PrefabUtility.ApplyObjectOverride` to apply modifications to the original prefab³.

5. **Debugging**:
   - Verify that the `myOutput` reference is correctly assigned in the Inspector.
   - Check if any other scripts or components are interfering with the text update process.
   - Use `Debug.Log` statements to track the flow of execution and ensure that the relevant code is being called.

Remember to test each solution individually to identify which one resolves the issue. If none of these approaches work, consider creating a minimal test case (without other complex components) to isolate the problem further. Good luck, and I hope you find a solution! 😊

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I've provided some potential solutions based on common issues related to `TextMeshProUGUI` not updating. If you need further assistance or have additional questions, feel free to ask! 🚀

Source: Conversation with Copilot, 6/3/2024
(1) Why is TextMeshPro not Updating textBounds After Setting text?. https://stackoverflow.com/questions/55417905/why-is-textmeshpro-not-updating-textbounds-after-setting-text.
(2) Text vertices not updating for TextMeshProUGUI - Unity Forum. https://forum.unity.com/threads/text-vertices-not-updating-for-textmeshprougui.1153142/.
(3) ,Change Text of TextMeshProUGUI using Custom Editor. https://discussions.unity.com/t/change-text-of-textmeshprougui-using-custom-editor/257923.
(4) Resolved - TextMeshProGUI not updating text - Unity Forum. https://forum.unity.com/threads/textmeshprogui-not-updating-text.1101559/.
(5) TextMesh Pro - Textmesh pro is not updating when changing text .... https://forum.unity.com/threads/textmesh-pro-is-not-updating-when-changing-text-programatically.490125/.