I'm increasingly experiencing a spontaneous situation when I am building a complex journey in Journey Builder, that the entry source definition (the green icon, top left), in my case a data extension, will disappear and force me to recreate it. This is happening multiple times in the course of building any 1 journey and costing hours of time.
In my case this also invalidates all of my decision splits and forces me to reconfigure them, since they each contain a rule that looks at journey data. They will revert from orange back to gray.
As a slight variation on this scenario, sometimes only the schedule disappears. However, the outcome is the still the same because you cannot recreate a schedule without first deleting and reassociating the entry source.
I can confirm that nothing is happening to the data extension, that wouldn't be the reason. And nothing is happening to the entry source definition automation either.