Severed Coriander Root Dying After 29 Days?

Something has happened to the coriander root severed 29 days ago. The severed half seems to be dying.

Sorry about the photo. But the severed base of the old root (red) has suddenly begun to wither at the top. Strangely, it still seems white at the base, presumably where it's attached to the other roots. However, what I don't understand is why the base of the new lateral side shoot (pink), attached to the original intact root, is turning brown at its base. Since it seemed so strong and healthy, I would have expected it to be pristine white.

Seems strange one is withering at base, other at top. i.e. Both are dying in roughly the same part of the cup. Could there be something (perhaps a toxin of some sort) causing this? I don't know. But what I can say is that there's no compost behind the roots in this area, so it might have caused too much desiccation (drying).

However, it's unlikely, since roots do not normally turn brown and begin to die when they reach an air pocket; they just stop growing and hang around in limbo. Mysteries, mysteries.

P.S. Have started another pot of coriander and have cut the roots in several places. Tell you about this later . . ..

P.P.S. Rising temperatures means now watering about every 5 days. Look at how much this watering has stimulated root growth.