Found three documents, found this place:

- (CGRect)textRectForBounds:(CGRect)bounds
     limitedToNumberOfLines:(NSInteger)numberOfLines
{
    bounds = UIEdgeInsetsInsetRect(bounds, self.textInsets);
    if (!self.attributedText) {
        return [super textRectForBounds:bounds limitedToNumberOfLines:numberOfLines];
    }

    CGRect textRect = bounds;

    // Calculate height with a minimum of double the font pointSize, to ensure that CTFramesetterSuggestFrameSizeWithConstraints doesn't return CGSizeZero, as it would if textRect height is insufficient.
    textRect.size.height = MAX(self.font.lineHeight * MAX(2, numberOfLines), bounds.size.height);

    // Adjust the text to be in the center vertically, if the text size is smaller than bounds
    CGSize textSize = CTFramesetterSuggestFrameSizeWithConstraints([self framesetter], CFRangeMake(0, (CFIndex)[self.attributedText length]), NULL, textRect.size, NULL);
    textSize = CGSizeMake(CGFloat_ceil(textSize.width), CGFloat_ceil(textSize.height)); // Fix for iOS 4, CTFramesetterSuggestFrameSizeWithConstraints sometimes returns fractional sizes

    if (textSize.height < bounds.size.height) {
        CGFloat yOffset = 0.0f;
        switch (self.verticalAlignment) {
            case TTTAttributedLabelVerticalAlignmentCenter:
                yOffset = CGFloat_floor((bounds.size.height - textSize.height) / 2.0f);
                break;
            case TTTAttributedLabelVerticalAlignmentBottom:
                yOffset = bounds.size.height - textSize.height;
                break;
            case TTTAttributedLabelVerticalAlignmentTop:
            default:
                break;
        }

        textRect.origin.y += yOffset;
    }

    return textRect;
}

In this place, yOffset of emojiLabel is recalculated according to verticalAlignment. If TTTAttributedLabelVerticalAlignmentCenter and TTTAttributedLabelVerticalAlignmentBottom are used, yOffset will increase. There is not much time to study why TT does this, but it is the problem caused here.

There are two modifications in MLEmojiLabel: 1. In the method of calculating cell height, use emojiLabel.verticalAlignment = TTTAttributedLabelVerticalAlignmentTop when creating static emojiLabel; 2. Use TTTAttributedLabelVerticalAlignmentTop when creating emojiLabel in your own cell. Using TTTAttributedLabelVerticalAlignmentTop will no longer recalculate the yOffset of emojiLabel.