what you have described is what is happening to the roots of one of mine. Unfortunately it has been going on for months and now the plant is starting to suffer.
I didn't pay much attention to it because I thought I had done everything I could do for it and I actually bought 2 at the same time. One is doing great, the other one is not so if one doesn't make it I still have the other one but your post made me think and have a closer look and I discovered something...
I don't know how I missed this when I re-potted it but look what I found hidden away this time round:
It's been suffocating the roots.
I would highly recommend you get a very light and airy medium for your dendrobium.
I find clay pebbles work but they need to be wicking up water and stay damp constantly otherwise the roots dry very fast.
coco does not work long term in my experience - it doesn't let enough air in.
Bark is good, add in some perlite and you have a great airy mix.
I wish I had checked the roots sooner instead of thinking I had already repotted it - in my defence it was one of my first orchids but looking at the picture I didn't even rince the old coco off the roots before repotting which is something I definetely should have done - the plant would be looking a lot better by now