Clematis, beauty and problem

I grow a few clematis, and for the most part they make me happy -- when the deer don't prune them that is. The one that I grow on my mailbox has performed quite well for me, especially considering how shady it is here.

I believe I purchased this as Clematis 'Piilu', but that's almost certainly not what it is. It looks so good with the variegated bamboo (Sasaella masamuneana 'Albostriata'), doesn't it?

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It's really quite nice regardless of the unknown ID...

...but it does have a problem. Every year part of the vine dies before the flower buds come out.

The first year it happened I thought it was damage from deer or an insect. Now that it happens annually, I'm really not sure what's going on.

Here's a view of the whole plant:

(I really need to replace the red zip tie that's holding up the cage/trellis!)

At least this year there were plenty of blooms left to help "hide" the problem.

Any ideas of what's going on and how I can prevent it next year?

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