I have experienced this on 3 Macbooks since the Sonoma update. First, with my 2021 14" MacBook Pro M1. Within days of the update, the Magsafe charger (which had worked without issue since purchase) started flashing red. I spent 2 months making repeat trips to the "local" (3-hr drive roundtrip) Apple store to diagnose and repair. They sent me home repeatedly with new cables only to have the problem repeat itself within 2-3 days. The charging block worked just fine in-store and at home with USB-C cables, but the Magsafe cable seems to burn out with limited use, even when fresh out of the box from the Apple store. Their team went so far as to replace the MagSafe board in the unit itself, to try and remedy. But meanwhile I bought a brand new 2023 14" Macbook Pro M3 to try and just replace and eliminate the problem. The same thing happened yet again, 3rd Macbook. Simultaneously, my office shipped me a 16" 2022 Macbook Pro. Once updated, same thing. The USB-C cable works in all 3 (as do the original, in-box power blocks). But MagSafe is useless. The manager of the local Genius Bar told me to return my new M3 unit while I can if they do not release a firmware update within the return window. So far, I see no indication that Apple is working to resolve this issue. Be warned.