Fun thing about the autosave feature is that if you’ve got your project into some precarious state where it’s possible to trigger a bug that will crash the app and you waited just long enough before actually pulling that trigger - then when you recover the project, there’s a pretty good chance you’ll be right back in that state again - so performing the same action again will trigger the bug again, lather, rinse, repeat …
I got into a loop like that with a bug in the time remapping (cf. Time remapping crash with 24.02.2), and it took a few goes to figure out what was really happening and how to break that cycle.
So another interesting test if it happens again might be to move a few things around with a different tool, or do something different, then try the spacer tool again. Which I suspect is probably what happened one way or another when you changed the appimage version you were using, and it just took a while to get whatever the real cause is to align badly again and set up the conditions needed to manifest the bug.
Which might help you work around it a bit, but it would still be really nice to narrow it down enough to find and fix it properly. It might be a rare corner case, but you do appear to be proving it’s something repeatable about your project and not just random corruption causing this.