Excel automatically undid changes

HelenaDavidson exactly the same thing has just happened to me. It was more of an issue for me because this was an "auto-save" SharePoint file, so I couldn't just elect to not save it. I also had problems with restoring a previous version (error message along the lines of I needed to save unsaved changes first. I saved it, closed it, re-opened it and tried to restore again but same message). In the end, I went back to an earlier copy with a different name and re-started from there. I was able to reproduce the behaviour by repeating the amendments but eventually cleared it by saving a fresh copy immediately before taking the action that caused the circular reference, which I then resolved, like you, by pasting values. The other feature I noticed, after the repetitive "Undo" of all my work, was that some formulae had not re-calculated (I had to go into editing the formula and press enter before it re-calculated). All very mysterious and presumably a subtle bug within Excel. Sadly, no one else seems to have encountered it yet, so little chance of a fix but truly very scary! Like you, I'm a very experienced user of Excel and I'm sure it wasn't just a silly mistake by me. The only thing remotely like it I've ever experienced is when Windows decided that it thought I was repeatedly pressing the down arrow key, which was only recovered by pressing the power button long enough to force a shutdown. Possibly a driver error. I toyed with the idea that this repeated Undo might be a repeated CTRL Z being processed but there's no evidence that it was still repeating after finishing all the Undo actions and finding you with the same experience after the same actions very much suggests an Excel bug.

My About info shows: Microsoft® Excel® for Microsoft 365 MSO (Version 2209 Build 16.0.15629.20196) 32-bit. Not sure why 32-bit as it's always been auto installed on a 64-bit system.