Constraints not working when rotating to different orientation

In storyboard, I have a UICollectionView whose height needs to be different when in portrait mode and when in landscape mode.

So I have two different height constraints on it, one for each size class:

----- and -----

The separate height constraints look like this (they use a multiplier to set the collection-view's height relative to the height of the super view):

--- and ---

When the app first runs, the constraint works (whether the app starts off in portrait or landscape). However, the problem is when rotating the device to the other orientation: nothing changes. The constraint for the other size class does not take effect, and the view remains the same size it was when the app first loaded.

I am unsure as to why my size independent size-constraints aren't kicking in when the device is rotated.

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