MacBook losing charge while shut down

Solution 1:

Even if your Macbook isn't drawing power from a battery, the battery itself is chemical in nature and, like all batteries, subject to "self-discharge". The only time this is an issue is when it's "elevated self-discharge." So your Macbook needn't be powered-on therefore to suffer some degree of battery drain.

So battery drain in a powered-down state isn't the problem- it happens to all chemical batteries. Your issue is that it's at an elevated level, which might indicate the battery requires replacement as the 2nd link talks about. Apparently 5% loss within 24 hours and 1-2% per day thereafter is expected for Lithium batteries. Yours is definitely draining at a much higher rate.

If you go to the "Power" section of "System Report" you can check the status of your battery to see if it's misbehaving as mine apparently is in my 2018 Macbook!:

So assuming you're 100% sure that the affected Macbook isn't in a sleep state, but fully powered down, this looks a lot like a hardware issue regarding the battery. Here's some guidance from Apple on troubleshooting Battery issues

Another thing you can look at for the sake of completeness when reviewing your power issue is resetting the Service Management Controller (SMC), if only to exclude this as not being causal before you trundle down to Apple with your Macbook-