Hi @georgeab,
Sorry to hear about this unusual zoom issue. Could you please confirm the version of Illustrator and your Windows OS details? It would also help to know if this happens with a specific file or across all files, and whether the artboard dimensions remain changed after restarting the app. If possible, please share a short screen recording of the behavior, so I can review it further. In the meantime, try testing the issue in Safe Mode to rule out any third-party interference: Start your PC in Safe Mode.
Looking forward to your update.
Abhishek