Reproduction steps:
1. Create a new project
2. Create Text - TextMeshPro GameObject (GameObject > UI (Canvas) > Text - TextMeshPro)
3. Enter long text into Text field in the Inspector
4. Change Font Size value to 1
5. Change Overflow to Ellipsis

Actual result: “IndexOutOfRangeException: Index was outside the bounds of the array” error is thrown
Expected result: No warnings or errors are thrown

Reproducible with: 6000.0.67f1, 6000.3.9f1, 6000.4.0b9, 6000.5.0a7

Reproduced on: macOS Sequoia 26.2 (M4)

Note:
Full error:

IndexOutOfRangeException: Index was outside the bounds of the array.
TMPro.TMP_Text.FillCharacterVertexBuffers (System.Int32 i) (at ./Library/PackageCache/com.unity.ugui@bb329a87fcdc/Runtime/TMP/TMP_Text.cs:5529)
TMPro.TextMeshProUGUI.GenerateTextMesh () (at ./Library/PackageCache/com.unity.ugui@bb329a87fcdc/Runtime/TMP/TextMeshProUGUI.cs:4914)
TMPro.TextMeshProUGUI.OnPreRenderCanvas () (at ./Library/PackageCache/com.unity.ugui@bb329a87fcdc/Runtime/TMP/TextMeshProUGUI.cs:2522)
TMPro.TextMeshProUGUI.Rebuild (UnityEngine.UI.CanvasUpdate update) (at ./Library/PackageCache/com.unity.ugui@bb329a87fcdc/Runtime/TMP/TextMeshProUGUI.cs:229)
UnityEngine.UI.CanvasUpdateRegistry.PerformUpdate () (at ./Library/PackageCache/com.unity.ugui@bb329a87fcdc/Runtime/UGUI/UI/Core/CanvasUpdateRegistry.cs:215)
UnityEngine.Canvas:SendWillRenderCanvases() (at /Users/bokken/build/output/unity/unity/Modules/UI/ScriptBindings/UICanvas.bindings.cs:121)