It's not a Windows button; it's actually just a sprite. What's strange is that my save-game screen in the same project uses most of the same events, and it works just fine. I'll show you what I mean.
Here is the event layout. I deleted everything irrelevant for purposes of this example while keeping the same functionality. There's nothing else in the layout; it ends at the bottom. Here's how it looks, but does not work (the saved array ends up all zeroed-out). The important bit is highlighted in yellow:
And here, the only difference is the important lines (yellow again) were moved to the start. This one works:
(The Timeline bit doesn't make a difference; that was just leftover from me trying something. It behaves the same either way.)
For the life of me I can't figure why clicking on the sprite would wipe the array out.