My clematis, like yours, has some parts going brown and dying, despite this summer, having loads and loads of flowers. Have made sure the garden was watered on dry days. Could it be ants? Some of petals are curling.
I have another clematis at the front, in full sun, which I put into a bigger pot and it had loads of flowers, but now something has eaten every single leaf! Can't see any insects. This one has smaller pink flowers.