That was certainly a quick turnaround. Having released Patch 1.3.3 yesterday for Returnal, that was shortly pulled by Housemarque, as it became apparent 1.3.3 was corrupting save data for a large number of players, leading to a formal apology from the developers.
Thankfully, Patch 1.3.4 has just gone live, which reverts back to the previous 1.3.1, but still fixes a trophy issue. Notably, Housemarque also state they've taken care of the keyboard exploit - the one that allowed players to use developer cheats - making that rather short-lived. You can find the full patch notes on the official website, but here's what Patch 1.3.4 does.
Returnal Patch 1.3.4 Is Now Live, Fixing Previous Save Issues
Patch 1.3.4 Notes
- Some game saves will have become corrupted with Patch 1.3.3, but maintained their file size. Players with those save games are able to safely continue their progress after installing Patch 1.3.4 without the need for using a backed up save game.
- Unfortunately other save games will have been effectively "deleted" by Patch 1.3.3, with their file size reducing to 0KB. These save games are not directly salvageable by Patch 1.3.4, without a backup save game available.
- In those cases, please try using a backed up save from PS+ if available, after installing Patch 1.3.4. With an intact save game from PS+, players can safely restore and continue playing with Patch 1.3.4 installed.
- Depending on the console settings, the cloud save may have been overwritten by the "deleted" save game noted above. If this is the case, progress will remain unsalvageable, even with Patch 1.3.4.