Tab is actually a non-printing character—or rather, a control character. What it does is entirely dependent on the application. What exactly do you expect? 8 spaces? 4 spaces? As many spaces as needed to get to a multiple of 8 columns? Indentation of the following text by one cm?
To put it short: The Label
control doesn't support tabs. Actually, Label just uses normal graphics routines for rendering its text and how should they know what you intend to do with your tab character?
If you need to display that character as a number of spaces, then you should replace it by that number of spaces.
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