I got a similar problem after upgrading to Big Sur too when the battery goes around 50%. It's like the OS is not reading the battery status correctly when I log on and thinks the battery is empty when it's still at 50% giving me the empty "battery / plug in" logo when I try to get it out of sleep! Eventually I can get it out of sleep and can continue my work!!! Unfortunately, the upgrade os by os trick didn't work, and Apple doesn't even know what to do or tell me. It's like Big Sur is breaking the battery firmware somehow! Weird enough, when the battery is between 100% and around 50%, I have no problem and the macbook starts with no problem, I can even use it until the battery is really empty. Those battery bugs are all too weird and should be adressed by Apple, tho I think that Intel based macs are not a priority anymore!