Made camera animation in timeline, but can't add keyframe now

Please have a look at this video
(19) timeline bug - YouTube

I have made the animation in timeline, convert to clip. no new property added. Now I just want to come back and edit it, with record mode on. But any change can't be keyed. What do I miss?

temp workaround is I won't use the converted clip > delete it from timeline > record another camera anim > copy the keyframes from the old clip.
It must be something wrong. But I can continue the work now.