So, this is the complete saving function I did.
As you can see there is first a call to a save level function with callback.
Once this is finished, in the callback function, I run the save game instance with call back.
As I mentioned before, this was not saving the game instance file.
If I then disconnect the save level and connect directly the save game instance, the save game instance is correctly saved.
So, it seems that 2 savings (one for the level and one for the game instance) one after the other will not work, doing only the first saving but not the second.
Maybe this is not acceptable by your plugin concept.
I did a last test with a small delay before calling the second save (Game instance), and in this case seems to work. Both file are saved!
Another small issues I found, is that if I run a saving with the callback, creating the new slot instance before it, the filename is not the one inserted in the “Slot File Name” field, but it fall back to the slot Asset name, even if it is not empty…