When a starsytem has been explored and the spaceship has left the system, the system returns to unexplored. no information is visible anymore, no planets are visible.
That is being addressed by the developers somewhat soon I believe. It’s been an issue for quite some time. It’s not a bug. Once you are out of sensor range, the information disappears, when you return, the last information you had returns (or should). Most of us keep the info written down for now, or leave a scout behind until you can get back there to perform whatever task you wanted to there (picking up res or colonizing).
It’s a feature, heh.
I suggest spreadsheets, or lots of screenshots.