This is expected behaviour atm (unfortunately). This topic goes into it a little deeper:
The tldr is that tracks are the same as layers. If you place two assets in the same position and timeframe on the same layer the edit API will try to show both at the same time. Normally this isn’t an issue in GUI editors as it won’t allow you to place a clip on the same layer during the same time period.
The current studio editor layout is unfortunately not helping as it doesn’t show there is an issue in the preview. An upcoming update will introduce a timeline based view that will make it impossible to get these types of results.
Flashing behaviour
When placing two assets on the same track during the same time the resulting video will flash: