Microsoft Defender - Big Problems on Big Sur

Read about the built-in security of macOS. Anything that Windows Defender would attempt to detect on Windows does not run on macOS (including viruses, trojans, etc.), and couldn't infect the operating system because it is on a code-signed, read-only drive partition.

The one thing that Windows Defender, as do other anti-virus applications on Mac does well is to trigger false alerts of legitimate application and system components and interfere with the normal operation of macOS. Decades of posts in these communities as evidence of that negative. Running any anti-virus product may satisfy an IT Security policy, but will do nothing for the end user operating system experience as delivered by Apple.

We have a sister company that's also testing Big Sur and they're not experiencing the same troubles and problems as we are. Has anyone else experienced similar troubles, or have any guidance on where to resolve?

Do you have any other system modifications installed that your sister company does not.

Usually, having two different experiences would imply that you have something installed that they do not, but when it comes to AV software, it may just be the software. They are so poorly designed, what works on one computer will not work on another identically configured computer.

If you need to scan for Windows malware, perhaps ClamXAV would be a better choice for your company. However, that recommendation usually goes for using it without its real-time scanner running. I don't know how invasive it is with that enabled.

What exactly is the purpose of Defender in the first place? Like all similar software, it serves merely to cause problems without providing any benefit.

The ClamXAV real-time scanner is highly rated by independent testing for not being invasive, however, in order to use it to scan for Windows malware, one would have to make certain that the Advanced preference "Enable detection of non-Mac malware" is checked which significantly increases the signature database along with the time required for manual/scheduled scans.

If the user is also running Windows on their Mac, it is recommended that the Windows VM be excluded from ClamXAV scans and a separate Windows based AV utility be used therein.

TL;DR: the protection provided by ClamXAV against Windows malware is strictly to prevent such malware from being passed along to Windows OS users.

Could not agree more to @ Barney-15E. These types of software interfere with the OS