All (!) other game than KSP saves constructs players create globally or in another folder, or something like that. To be able to transfer constructs between saves and to avoid erasing constructs by mistake.
The system KSP uses means that it is IMPOSSIBLE to copý your own ships between saves. And if you erase a save the ships you have created is erased too.
I just did that in KSP2. I had a save that was bugged (with rocket scrap that could not be erased all over the save) and when i erased it I erased my ships.
Save them globally, or in a "ships" folder ingame, instead, not connected to the saved game.