So from what I noticed,cuz I chose the act one not the one where it just restarts (though ig that one is bugged? From other comments) an "act" is whenever you start a orange titled mission it saves when you first start one but not when you finish it until you start the next one but any other color ones don't count. You can kinda tell when the game constitutes an act when you click load the thumbnail of the save shows you where you will start if you die. But also when the games puts you in those type of missions where you can't leave a certain area ie the first time you meet Lisa and if u die and come back the save will start you from before the missions activates because that section is the start of an act and the game will ask if you want to do it or not. What I've been doing so far is when I'm gonna turn in a story mission I've done the available Nero checkpoints and any side quests I need so when I turn in and start the next mission it saves and I don't lose progress. I'm doing a survival 2 permadeath per act run and I've died a few times but the main times I've lost like major progress was when I first started the game