Hello Julien,
By design when the Start and End time of RadGanttView task have the same value a milestone is visualized - that is a built in functionality of RadGanttView and could not be disabled. What we could suggest you in order to prevent that visualization is to set values for the Start and End time of the tasks with a small difference between them (a few seconds or milliseconds) - thus the milestone would not be visualized.
Hopes this helps.
Regards,
Nasko
Telerik
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