MacBook M3 shuts down suddenly at 40–50% battery, shows 0% on restart

Have you tried to see If this behavior also occurs in the safe mode?

Start up your Mac with Apple silicon in safe mode

1. On your Mac, choose Apple menu  > Shut Down.
2. Wait for your Mac to shut down completely. A Mac is completely shut down when the screen is black and any lights (including in the Touch Bar) are off.
3. Press and hold the power button on your Mac until “Loading startup options” appears.
4. Select a volume.
5. Press and hold the Shift key, then click Continue in Safe Mode.
6. The computer restarts automatically. When the login window appears, you should see “Safe Boot” in the menu bar.

If the issue does not occur in the safe mode, it is caused by an external piece of software

Otherwise it could well be a hardware issue. I have a M3 MacBook Air via my work and that one is not having the issues you're describing

if you really did run your computer from Noon to 10 pm on battery, the likely issue is that your "gas gauge" is somewhat less accurate than most.

The Approved Method for improving battery estimates, is to charge your computer until it indicates 100 percent charged, then leave it connected to power for several more hours beyond Full.

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 will 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 maintains optimum battery charge levels under program control, and will NEVER over-charge. Activate Battery Health Management and do not spend another moment of your time thinking about 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 when nominally asleep.

I have seen a couple of M1 MBAirs where I experienced this issue (occurred with both third party & OEM batteries IIRC). Unfortunately on my initial investigation revealed nothing since no logs survived during that time period. I have yet to revisit those laptops because of the huge amount of time involved to figure out a method of monitoring them closely. With one laptop my custom script reported a significant charge on the battery which dropped to 0% charge within five hours (I tried to access the laptop in the morning & five hours earlier is the last entry showing significant charge....laptop had been sitting idle for days).

I suspect a bad battery even though I don't see any other signs. I had a special script I was running at the time to monitor the battery, but unfortunately it did not include full snapshots of all the battery information. That is my next step.

However, it is possible the system was still running, but everything stopped responding so no log entries were created during that time. Or some other odd hardware issue caused the battery to drain suddenly. Batteries are usually the weakest link here, but surprised to see it occur with several different laptops and a mix of OEM & brand new 3rd party batteries that otherwise tested perfect.

Not quite sure what to think. I've seen a few other posts on this forum where their battery graph showed sudden drops like yours.

There is no easy way to troubleshoot such an issue when the battery condition and Apple Diagnostics are not reporting any issues.

Your graphs suggest you did not plug in your Mac from Noon until after 10 PM, and at that time it suddenly shut down.

Is that correct?