Drag handle for corner radius should work for single corner when in "4 values" mode
In the sidebar you can toggle between changing a corner radius for all 4 corners at once, or to specifying the 4 values separately.
Additionally, on a rectangle itself, there is a dot in each corner that allows you to change the corner radius.
When in single-value mode, dragging a dot changes all corner radii. I would expect then that when you're in "4 value" mode in the sidebar, that dragging a single dot only affects that corner.
As we haven’t seen any activity on this issue in three months, I’m marking it as inactive. Please do feel free to file it again if you’re still experiencing it!
AdminCorey Lucier (Adobe) (Admin, Adobe) commented
No the on-canvas editing controls work consistently regardless of context (what is focused in the PI/sidebar).
You can drag the corner "dot" with option key down to affect only one corner.