Example to centre on cx and cy:

private final Rect textBounds = new Rect(); //don't new this up in a draw method

public void drawTextCentred(Canvas canvas, Paint paint, String text, float cx, float cy){
  paint.getTextBounds(text, 0, text.length(), textBounds);
  canvas.drawText(text, cx - textBounds.exactCenterX(), cy - textBounds.exactCenterY(), paint);
}

Why doesn’t height()/2f work the same?

exactCentre() = (top + bottom) / 2f.

height()/2f = (bottom - top) / 2f

These would only yield the same result when top is 0. This may be the case for some fonts at all sizes, or other fonts at some sizes, but not for all fonts at all sizes.