No, this is not a known bug. I logged it in our feedback portal - you can vote for the item and track its progress here
. I have updated your Telerik points for bringing the problem to our attention.
For the time being, the only workaround that I can suggest is to create a custom template for the RadTimeline and in that template, find the TimelineAnnotationsPanel and set its Grid.ColumnSpan to 1. For example:
You can find information on modifying ControlTemplates in our online documentation
Please note that this workaround has one drawback - the strings of the interval groups will be clipped. I have attached a screenshot to demonstrate this (note how the December string is clipped in the upper right corner).
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