One thing that confuses me is that the size of `Mega Factory.zip` (the corrupted one) is 8.29 MB while `Past Mega Factory.zip` (the uncorrupted one) is less. If `level.dat` was supposedly cleared, and locale is the same size, where did the 7 MB that were previously in `level.dat` go?

I was unable to reproduce the issue, and with what you said, it would make sense that mods were not the cause of the corruption. That leaves either external corruption outside of Factorio or corruption during that saving process of the world pre-activation of the mods. I've noticed that level.dat is ...

My bad for not clearing up the circumstances: The mods were enabled when the save got corrupted (when I went into "Load game" menu), but I disabled them after the corruption was reported to see if the vanilla save was still considered corrupted (and it was). The screenshot was taken after disabling ...

I installed the following mods today in order to create a new world:

Mods1.png
Mods2.png

I went into "Load game" to load my newly created modded world when it said the vanilla save had been corrupted. I hadn't opened the vanilla save in the modded client.

Attached is the vanilla save and a ...