This just happened again. This time without a crash.
I renamed a heading and after the rename the editor got sluggish, so I quit Obsidian as this fixes the problem). Obsidian UI quit, but the icon was still visible as an open app and a huge chunk got deleted.
I believe this is the same problem that causes the heading rename corruption Heading rename deletes arbitrary data, inserts garbage around heading links - briefly after a heading rename, there’s a race condition on the active buffer.