Ok, I have some significant findings.
1) This is tied to the STMMaskableGraphic class. When I deactivate or remove it, the exception does not appear.
2) As you suspected, this has something to do with a mismatch of the font which is selected in the Inspector properties of the STM object and the font which is assigned via tags. I am not sure what the mismatch is or what makes things compatible. All of the fonts I have created are OTF fonts and are compatible. But I also have some OTF fonts which are NOT compatible, and make the bug happen.
3) After text auto-typing completes, the shadows will vanish. This happens 100% of the time, whether or not compatible fonts are chosen.
3b) Commenting out ShowAllText() on the last line of ReadOutText() corrects this, so ShowAllText() must somehow eliminate the shadow glyphs
3bI) buuuut.... commenting this out also leads to another error entirely being thrown repeatedly (included below)
3c) Calling SkipToEnd() does not activate this bug
4) Dropping text in which does not auto type, but which appears instantly (for instance, editing it through the inspector) makes shadows appear and stay
4b) this is likely because ShowAllText() is not called in this case
EXCEPTION POPPING WHEN ShowAllText() is commented out at the bottom of ReadOutText():
ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection.
Parameter name: index
System.Collections.Generic.List`1[T].get_Item (System.Int32 index) (at <cac1dea318b4499fb9eaf77618617c23>:0)
Assets.Scripts.TestActorVoice.CreateGUI () (at Assets/Scripts/Editor/TestActorVoice.cs:38)
System.Reflection.RuntimeMethodInfo.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at <cac1dea318b4499fb9eaf77618617c23>:0)
Rethrow as TargetInvocationException: Exception has been thrown by the target of an invocation.
System.Reflection.RuntimeMethodInfo.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at <cac1dea318b4499fb9eaf77618617c23>:0)
System.Reflection.MethodBase.Invoke (System.Object obj, System.Object[] parameters) (at <cac1dea318b4499fb9eaf77618617c23>:0)
UnityEditor.UIElements.DefaultEditorWindowBackend.Invoke (System.String methodName) (at /Users/bokken/build/output/unity/unity/ModuleOverrides/com.unity.ui/Editor/WindowBackends/DefaultEditorWindowBackend.cs:358)
UnityEditor.RetainedMode:UpdateSchedulers() (at /Users/bokken/build/output/unity/unity/ModuleOverrides/com.unity.ui/Editor/RetainedMode.cs:55)