Regression introduced in R1 2021 SP1 results in an appearance glitch:
The Timeline generated two duplicate span elements with the same ID:
<span id="ctl00_ContentPlaceholder1_RadTimeline1_WebServiceCDS" style="display:none;"></span>
...
<span id="ctl00_ContentPlaceholder1_RadTimeline1_WebServiceCDS" style="display:none"></span>
This can be seen in the Overview demo: