Update: 7:44p Pacific
No Man's Sky might not be overwriting discovering after all. In an edit to his original post on game's sub-Reddit, Dark_Nexis explained that he redownloaded No Man's Sky and, after a couple of hours, noticed that the discoveries he'd made on his starting planet had reappeared.
Based on his anecdotal evidence, the issue might stem from game servers taking more or less time to refresh discoveries as players move further and further away from them. "It seems like a server bugs like it takes a while for your old discoveries to re download," he wrote. "Please spread the word and if you are having this problem just wait and see if your discoveries will return. It seems a little bugged where as you get further and further away from your starting point you can't see your old discoveries even when you find your way back. It's like the server lags behind big time and takes a while to find the old information."
Original story follows
According to several users on Reddit, No Man's Sky deletes any discoveries other than names of planets and solar systems after two weeks (via Geek.com).
Reddit user Dark_Nexis returned to the system where he began his adventure only to find that the names he'd given locations, flora, and fauna had been reset. "I check a few neighboring stars I discovered and their discovers were wiped. So none of my named animals plants were ever saved. The system name and planet names were saved but everything else was wiped." [Comment edited for grammar and spelling.]
Other players experiencing the same issue chimed in. "Yesterday I browsed my list of discoveries, and went all the way to my starting planet," wrote Ultrasilvanus. "The system and all the planets I've ever been to are clearly marked as being discovered by me, the planets list 100% completion for animal scans but I can't see any animal or plant in the gallery and all of them appear as ???????"
Further investigation revealed that No Man's Sky seems to be erasing discoveries in chronological order, from oldest to newest.
In defense of Hello Games, deleting discoveries could be a bug rather than a design decision. All evidence has been anecdotal so far. However, it's not surprising that affected players are disappointed. Whether a bug or design decision, No Man's Sky is predicated on exploration, discovery, and putting your mark on those discoveries by naming them—planting your flag in soil untouched by others, so to speak.
Without that hook, many players might decide that exploration is meaningless.