Cannot add or remove keyframes on Vibrant Visuals graphs (v26.30)

[Imagenum]

In the Vibrant Visuals Settings panel I can select and edit existing keyframes, but there's no control to add a new point or delete one.
Repro

Open Vibrant Visuals Settings, pick Atmospheric Scattering, then Sky Zenith Color.
Click an existing keyframe. I can change its time and color and drag it. That works.
Try to add a new point to the curve, or delete an existing one. Nothing does either.

Expected vs observed

The keyframe format supports an arbitrary number of points. The official Key Frame JSON Syntax docs show one setting using seven (0.0, 0.25, 0.35, 0.5, 0.65, 0.75, 1.0). So I expected to add and remove points on the graph. Observed: only the existing points are editable, no way to add or delete.
Other info

Build: 26.30
Frequency: Always
Setup: single player test world
GPU: RTX4090
Device: Win 11 Pro, Intel i9-13900KF

Editing the JSON directly allows more points, so this is an Editor UI gap, not a format limit.