Also reported by Neowin, the KB5121003 update is apparently also causing an issue on Lenovo laptops where AC power is randomly being disconnected. That’s a huge inconvenience for laptop owners when they need to charge up, yet neither Lenovo nor Microsoft have commented on or acknowledged the issue. Notably, rolling back the KB5121003 update seems to make the problem go away.
If you’re dealing with unexpected system instability or other problems after installing update KB5121003, your best bet right now would be to roll back the update and wait for future fixes.