Yes, Apple released a Big Sur 11.6.3 Wednesday Jan 26, 2022
Any Third Party Applications that will interfere with the normal operation of the OS, alter, modify, remove or delete or attempt to do so is an invitation for disaster and may require a Reinstallation of the OS.
This includes AntiVirus, Disk Cleaners, Disk Optimizes, UnInstaller etc.
This will include CleanMyMac
This will include BitDefender
The The Built in Security is all that is required.
There are factors:
1 - Like poor Wifi connection
2 - Slow wifi connection
3 - Too many other people downloading at the same time and Apple Severs are overloaded
4 - Software on the computer interfering with the download process including AntiVirus Software
5 - Running behind a VPN which may increase Packet losses and may corrupt the download
6 - If via wifi too many Other devices using the wifi at the same time reducing the Bandwidth to your computer.
7 - Best Results are via Ethernet Cable to Router