Just because you put a Mac to "sleep" does not mean it will sleep uninterrupted all night. There are several settings within macOS which can affect this, with Power Nap being one of them. It is also possible you may have some third party software installed which is running.
Sometimes people may think they put their Apple laptop to sleep or even powered it off, when in fact the computer never actually made it that far because of some software interrupting the sleep or shutdown process. In fact with the 2018+ Apple laptops, things get even more complicated because connecting or even disconnecting the power adapter can wake or power on these laptops. Pressing any key on the keyboard or even the Trackpad can wake or power on these laptops. Same thing can happen with the laptops sleep sensor which detects whether the Display has been opened. It would not surprise me that some externally connected device could cause the laptop to wake or power on as well.
I was assisting a tech co-worker with a similar issue of the battery draining overnight. I discovered that their issue seemed to be related to iCloud syncing their photos overnight. The standard macOS system logs had no information in them so I had to resort to using the command line to retrieve the system logs. I had to scroll through thousands (probably hundreds of thousand) of repeating entries trying to look for something. It was an extreme long shot being able to pick the iCloud & photo bits from the logs. The co-worker still has not completely confirmed my suspicions, but they did narrow it down a bit (I think they turned off WiFi....it has been a while so I don't recall the details). All I know is that the macOS system logs are nearly worthless for troubleshooting these days since there are so many repeating entries making it hard to detect any entries containing the real clues. Plus many of the log entries are non-sensical, and can sound very scary & dangerous....most of the entries are just plain non-sense and should be ignored. It just takes experience to know which entries are possibly important.
When sleeping the laptop, first press the Caps Lock key so its LED lights up. When the LED goes out, then the laptop is sleeping. Close the lid of the laptop to minimize the chances of a key or Trackpad being pressed accidentally. Perhaps before sleeping the laptop, disconnect all external devices in case one of them is causing a problem. Could your laptop be performing backups or updates? Maybe turn off WiFi before you sleep the laptop to see if that makes any difference....if this works, then look at iCloud if you have it enabled, or any other cloud based file syncing services you may have.
More than likely you have a software or configuration issue. Figuring out what software or setting is the problem. Keep in mind the co-worker helps support our organization's computers, but they came to me to verify the hardware and to offer advice, so it shows even people who support computers every day don't always know how to figure some of these issues out. I was only able to point them in the general direction.