Green orchid root tips turn non-green

[Ross]

This question regards several orchids in varying culture regimes (terrarium, non-terrarium) whose previously green root tips have ceased being green and end up just white like the rest of the root. I have one orchid (Amesiella monticola) in an orchidarium, whose previous fat warty white roots with new green tips, ran up against the continuously damp (not wet) sphagnum, then quit growing. They are not rotted, just blunt like something ended the growth. I have an Angraecum dideria in a mesh pot with bark and it had previously active growing roots (same fat warty appearance) that have started to lose the green end. Again, no signs of rot, drought, chewing, etc. I have a Sophronitis cernua with new active growth and one has a new root with lotsa green on end other growth had new root, but the end has stopped growing, but is still white. In all cases the roots get the green color you would expect after watering, it's just that they misteriously quit getting longer.

Is this all normal, or is it related to some cultural issue? Anyone else run into this before? I have a call in to Andy at Andy's Orchids, but he pulls plants for shipping on Wednesday and isn't available, so I thought I pick you all's brains