Some of the roadmap items in Roadmap Timeline Report show as bars, some as diamonds and some circles.
Release : Any Supported Release
This is working as designed, some items don't have a start or an end date, and some do. Here is the documentation explanation for this:
Items on the timeline can be bars, circles, or diamonds. Bars result when you enter both the start date and finish date for an item. Circles result when you enter only the start date for an item. Diamonds result when you enter only the finish date for an item.
Broadcom Clarity documentation: Roadmap timeline display