Game save just doesn't consistently work. I've gone from having the first four tiers all bought to having next to nothing twice now. Uninterested in further attempts at playing.
It is ambiguously phrased as to what the "stacks additively means". Individual idle coin shop category upgrades stack collectively.
Which means everything except the most expensive one in each category is worthless unless you have left over idle coins and no means of earning more in game.
It does auto-save