Peculiar behaviour when using TextBlock.TextEffects

I am facing an peculiar issue when I use TextBlock.TextEffects.

I have three TextBlock control with Text property as follows,
1. ti
2. fi
3. zi

I apply the following TextEffects to all three TextBlock,
TextEffectCollection textEffetcs = new TextEffectCollection();

TextEffect textEffect = new TextEffect();
textEffect.PositionStart = 1;
textEffect.PositionCount = 1;
textEffect.Foreground = Brushes.Transparent;

textEffetcs.Add(textEffect);
textBlock.TextEffects = textEffetcs;
From my understanding, this will make the second character invisible.
However I notice that it is not true for the following scenario.
1. If the character which I am trying to apply the effect is the character i.
2. and it is preceded by either character t or f
3. and the font family is Calibri
Then the text effects does not work.
Has anyone notice this?