Hi Forged, I’m sorry to hear this issue has affected your save. Thank you for reporting the details of your experience, I shall pass these onto the team.
I completely understand how frustrating and disheartening this issue can be for players and as you have probably seen through the link Lothaer kindly posted above, there is a known issues post where we are collecting information and workarounds here: [Known Issue] Rafts/Gyro Vanishing
The team are still investigating and working on this issue and while I know the workarounds are not ideal, I hope they may be of some help - thank you for also clarifying it has occured for you even with closing the application. Please let me know if that last save was created after immediately returning to the island and this may be another cause of this issue for players.
Though it is also not ideal, it is possible to back up saves externally on a PS4 if you wish to do so. This may help to avoid this issue or allow you to revert to an older save while the team continues to work on this.