Multiple Timeline Tracks disappear when first track is collapsed

When collapsing a timeline track, if there are over 5 tracks below it, some will disappear.

This is a closer look at but UE-18550

Workaround
Simply scroll on the timeline tracks to repopulate the other tracks listed.

This is not a regression since this comes from a much older bug that was backlogged.

1. Open new blank project
2. Create new Actor class Blueprint
3. In the Event Graph, place a new Timeline node
4. Open the Timeline by double clicking the node
5. Add 5 new Floats by selecting the f+ button
6. Scroll to the top of the Timeline list
7. Collapse the first Float Track by selecting the small triangle

Result: Tracks 3 and 4 disappear

Expected Results: Timeline tracks do not disappear