I have seen this happen multiple times in Jira Product Discovery.
When I place an idea next to another in the timeline view, in specific spots of the timeline only, it won't let me add the idea there, when dragging it to the new location the idea moves or disappears automatically, and changes duration.
See the following video for a demo of this, when dragging the idea it moves automatically.
Is there a place to follow if and when this will be fixed? It looks like a pretty big bug to me, as I cannot build my timelines as I want.