Make sure to try the charger in another USB-C port especially on the other side of the laptop if you have the four port model. Also try rotating the USB-C connector of the charging cable 180 degrees upside down (I know it sounds crazy, but the USB-C port may be half bad).
You can monitor the charging and discharging activity using the following command in the Terminal app which will save a copy to a file on your Desktop called "laptop-battery-charge-log.txt". The output will also be displayed in the Terminal window so to terminate the command you need to press Control + C. Maybe it will provide a clue.
pmset -g rawlog | tee -a ~/Desktop/laptop-battery-charge-log.txt
Unfortunately the Apple system logs are now worthless for troubleshooting any issue. Make sure you don't have something that is scheduled to run at night. Maybe your laptop is not actually sleeping. Or maybe you have a corrupt .plist preference file (perhaps "Battery", or "Screensaver", or something else similar that could be related to the old "Energy Saver" preferences or settings for setting timers & usage, etc.) which needs to be deleted and recreated. I'm not familiar with the newer preferences for Big Sur so I cannot be more specific or even tell you where these prefs are located except they should be located in the user's home user Library folder which is hidden by default in the Finder. Or maybe you just found some bug with that version of Big Sur. Make sure Big Sur is completely up to date with the latest patches.
You should also try a clean install by first erasing the whole physical drive before reinstalling macOS and testing the laptop thoroughly before you restore or migrate from a backup and before installing any third party apps. This is the quickest way to figure out if you have a hardware issue and/or a bug with Big Sur. Installing an older version of macOS may be able to determine which of the two is most likely.
As @ku4hx mentions it could easily be a bad battery or even some other hardware issue. I have actually seen an Apple battery stop charging at around 72% on some of the older Magsafe laptops when I'm charging them overnight. That also does not make any sense, but I have seen enough of these laptops do this to know it is not an isolated incident although I'm uncertain whether it indicates a hardware issue or some OS/firmware bug. I have seen some batteries only show a problem when the battery is at a certain charge level.
Something else to consider is whether you are accidentally turning off the electrical power to the laptop either by accidentally turning of the surge strip or the electrical wall outlet. Or maybe you have some other device connected to that electrical circuit that is causing a power issue that is affecting the laptop and this is only occurring at night. This is especially important if you do not live alone as you don't know what someone else may be doing without even realizing it.
I know from personal experience it can be extremely difficult trying to troubleshoot issues with these USB-C Macs.