UILabel in UICollectionViewCell wrong height - ios8

Question

I’m facing a trouble with a UILabel in a reusable UICollectionViewCell.
The label can be placed in the UICollectionViewCell frame in different positions.
It can have different font sizes and it must be maximum 3 lines (Truncate Tail).
In order to do it I remove constraints (potentially inherited from another dequeued cell) programmatically like this:
[self.lblPadName removeConstraints:self.lblPadName.constraints];
for (NSLayoutConstraint *constraint in self.contentView.constraints) {
if (constraint.firstItem == self.lblPadName || constraint.secondItem == self.lblPadName) {
[self.contentView removeConstraint:constraint];
}
}
And I programmatically add the center alignment in contentView, the distance from the bottom, the vertical and horizontal constraints using a priority of 500 for the vertical one (Content Hugging Priority Vertical is 1000 and Content compression Resistance Priority vertical is 1000).
Here how i add the vertical and horizontal constraints.
[self.contentView addConstraint:[NSLayoutConstraint
constraintWithItem:self.lblPadName
attribute:NSLayoutAttributeWidth
relatedBy:NSLayoutRelationEqual
toItem:self.contentView
attribute:NSLayoutAttributeWidth
multiplier:multiplier //0.92f
constant:0.0]];
NSArray *lblPadName_constraints_Height = [NSLayoutConstraint constraintsWithVisualFormat:[NSString stringWithFormat:#"V:[%#(%##500)]", kTitleViewKey, kTitleKeyHeight] options:0 metrics:self.metrics views:self.viewsDictionary];
[self.lblPadName addConstraints:lblPadName_constraints_Height];
In the storyboard i also set the preferredmaxlayoutwidth of the label and its maximum number of lines.
Each cell is provided by an handle user can use to resize the cell (The UICollectionView uses a flow layout).
The data source for the collectionView is provided by a NSFRC which has a delegate that reload the changed items when the model is updated.
In the cellForRowAtIndex the setup method of the dequeued cell is called and since now no problems: the text is correctly on two lines.
The frustrating problem comes out when I try to resize the cell and the system saves on CoreData the new size.
The NSFRC call its delegate, the UICollectionView reloads its data but the UILabel now has the height i passed as a constraint and the text is forced to be in one line.
It seems that the Content Hugging Priority Vertical and Content compression Resistance Priority don't affect the label's height.
I'm using iOS8.
How can I make the UILabel bound perfectly fits the internal text using constraints, Hugging Priority Vertical and Content Compression Resistance Priority?