How to fix 1% battery issue on MacBook Pro running macOS 26.2?

I recomimend continuing to pursue this with AppleCare. Be absolutely certain your Mac is fully shutdown, and not still on when you close the lid. I have a 2021 16-in MacBook Pro M1 Pro that I have to wait for the keyboard backlighting to extinguish before I close the lid. If not, it continues to run when I think it is off and that whacks the battery charge.

Since these are communities with voluntary assistance from fellow users, and no Apple employees participate here, there is little to nothing we can do about your battery discharge situation.

That computer is a battery-CAPABLE device, It is not optimized as a battery-operated device (it is not an iPhone.)

Your computer performs best when connected to AC power, such as the power adapter. It can use the full output of the Power Adapter AND when doing especially challenging work could also freely "borrow" power from the battery. In some cases, even with the power adapter connected, the charged state may decline during very stressful work.

When used only on battery, your computer has no extra cushion of power, and may perform more slowly. However, for ordinary non-stressful tasks this may not be objectionable (possibly not even noticeable.)

In general, you should ALWAYS connect a power source when it is possible to do so, and only run on batteries (which could be somewhat slower) when no power sources are at hand. Modern Macs maintain optimum battery charge levels under program control, and will NEVER over-charge. Connected to Power is NOT necessarily charging.

When you set it down in one place, or set it down for the night, Plug it in. Then you won’t CARE whether it would drain the battery.