If you click on the collapse/expand button of a group (when the data is grouped) while you add or remove an item from the ItemsSource of RadTimeline, all items disappear.
To work this around set the AnimationManager.AnimationSelector attached property on all TimelineItemGroupControl elements to null. You can do this using an implicit style.
The theme is not propagated to the TimelineStripLineControls when it is set on the timeline control with StyleManager. Which leads to the color of every second strip line using the default color for the Office_Black theme
The theme foreground is not applied on the group header when the theme is applied with StyleManager For example: The foreground for the Expression_Dark theme should be #FFDDDDDD, but instead it is #FF000000