For me it seems like when you spawn more bosses at once,you still only have the chance to get only 1 higher tier box (When I spawned 200 tier1 bosses,I got 0 tier2 boxes)
I don't know what to say. I don't have any control over what happens to save files... I can only make sure it works on the game's end. :| It's not an overly complicated code. One line(per variable) to create save, one line to extract info to save, one line takes extracted info to load game...
I just did more tests and it seems to work right.