Have you ever seen something like this when completing a level?
This is a bug that I haven't seen reported often, but basically, some players on new save files would have to beat levels twice, or in some cases, their save files wouldn't be accessed at all. Normally this kind of thing is affected by out-of-game factors such as firewall or Steam itself, somehow, so I figured it was a machine-dependent issue. However, turns out it's something that could affect everyone?!
Fast forward to today, and a player finally managed to establish a pattern: the glitch seems to occur consistently if you fully beat the tutorial on a new save, but not if you skip it, but only when the game is running with Steam open. Whether this issue cropped up recently is entirely beyond me, but that allowed me to eventually figure out that somehow, Steam's extra logic seemed to change the order of how the game's main code was executed, leading to the save system not knowing what level to save the score for at the end. I still don't understand how skipping the tutorial alleviated this issue, beyond the achievement unlock being what altered the code order. The code itself was basically never truly wrong but was instead vulnerable to an outside factor changing how it was executed. Really really weird stuff that was ultimately fixed by duplicating a single line of code.
In simpler nerdy terms: a race condition got spotted and fixed.
I apologize to everyone who's had to deal with this issue, because it has been a headache to figure out for months, but it should be resolved now!
Also sorry that this isn't a more substantial patch with more fixes, as I'm still focused on the next update stuff, but since this fix was enough of an emergency, I basically injected the necessary line of code directly into the previous build of the game instead of making a new one.