Kris,
mulch is great. Will Goodwin was a monster for clematis wilt for me. It comepletely covered itself with flower buds just to wilt the morning they were supposed to open. I had it for 5 years and saw only a couple of flowers to open. I finally shovel pruned it to avoid the heartbreak. It's too bad, 'cause I find the color and the ruffled edges of the the flowers just breathtaking. Sorry for the bad news, maybe it will behave better for you, but my guess is, it's wilt.
Grow

Cut the wilted stem off. The reason is most likely because it has three really long stems that the root system can't support. This form of wilting is usually outgrown. Hard pruning all Clematis when young helps to let the roots catch up to the top growth. They are really set back by transplanting.

I noticed the leaves of the wilted branch are brown, started to brown from top to bottom. Stems are still brown.

I heard that for fungal wilt , color will be black not brown for leaves-- is that always the case??

Yes I cut it out!!

Am I right ?

"I heard that for fungal wilt , color will be black not brown for leaves-- is that always the case??

"