Steps to Repro :
1) Create an empty timeline (that will be the main timeline).
2) Create a control track.
3) Create another timeline using another gameobject (that will be the subtimeline).
4) Drag and drop the subtimeline gameobject on the control track of the main timeline => that will create a subtimeline clip.
5) Right click and press "Match Content" menu.

Current Result : The clip disappears.
Expected Result : The clip stays the same.

Reason : As the subtimeline doesn't have any duration, the match content try to make the start/end of the subtimeline clip in the main timeline match the not existing value of the subtimeline => the clip disappears.

I think it's a regression but I don't know since when it began.