Brown ring on vanda roots

[Bunch_Of_Roots]

Hi all, I noticed my Vanda denisioniana developed brown rings on a couple of its roots. The tips of the affected roots would continue to grow. Then one day a majority of the root would die back but not the whole root. The problem doesn't seem to be contagious as other roots are not affected by it. And the problem appears randomly.

The vanda is cultivated in vase culture for a while now. And there are always active root growth. I currently soak the roots for about 8 hours, once in a while overnight, in the recommended amount of Dyna-gro orchid pro and RO water solution every 2-3 days. It receives a good amount of sunlight. My environment's night temp is above 60 F, day temp about 70 F. Thanks!

Here are some pictures for references:





This is another root that had the same problem. And this is how it looks after like a month.