Quick update :
After further testing I think I can confirm the issue is caused by switching profiles while the game is running. Tried switching and it didn’t save, closed the game then started it again and saving seems to work in both SP and Co-op.
Now I hope this will be either fixed (not selfishly because I know how to bypass this, but for others), or that it will be visible somewhere because it’s really gamebreaking.
While I’m here and to avoir creating a new topic, quick question : my boyfriend started the game a bit before me on my console (he’s the one I’m switching accounts with). I can see his base in my world, but it hasn’t been updated in a long time, probably since I started myself. Is this expected, and thus is it possible to share a world asynchronously (until we start co-op) ? We never played co-op together.