Speed time remapping keyframes become inoperable if last keyframe is deleted (see attached)
This bug occurs when the last speed remapping keyframe is deleted.
Add a bunch of speed remapping keyframes
Alter speed of clip etc
Delete keyframes (a subsequent one should appear with "frameless media" - clip extended beyond actual frame, see attached)
Delete this keyframe and it becomes impossible to add anymore keyframes even in the effect controls panel
If you don't delete that last keyframe but extend the clip to get rid of the "frameless" portion the keyframe disappears and allows keyframes to be added as normal
(Hope all that makes sense)