Tinderbox uses underlying Apple frameworks to automatically substitute a pair of successive dashes -- (as via keyboard minus key) with a long en-dash —. Note: for historical and practical reasons this feature is not used in aTbRef.
Smart Dashes are also controlled via $SmartQuotes. This makes it easier for those with more precise quote/dash control to disable these smart features which are on by default
Either change $SmartQuotes for the note or use the Substitutions menu either via the Edit menu or the text area's right-click menu. Warning: this menu change only lasts until the note is de-selected, on re-selecting the note the default value is re-applied.