Macintosh HD not mounted, System Snapshot mounted instead after Boot Camp partition failure

I am using a Late 2013 MacBook Pro 13" Retina, recently updated to Big Sur (11.0.1).

I tried to install Windows using Boot Camp Assistant. I ultimately ran into this alert: ”Your disk could not be partitioned. An error occurred while partitioning the disk. Please run First Aid from within Disk Utility to check and fix the error.”

After First Aid did not appear to find or rectify any errors, the next step suggested by Apple was to check for and reclaim free space. However, the Partition window returns this— with an un-clickable, solid blue quarter of the pie chart where I would normally expect the free space section to be:

I also noticed that Disk Utility was showing 'com.apple.os.update-779...' (an 'APFS System Snapshot') as a sub-drive under Macintosh HD, which is something I had not seen or heard of before. I read that the presence of the System Snapshot could be a reason for the the disk not recognising that there was free space sort of, therefore causing the attempted partition in Boot Camp to fail. (Disk Utility itself does recognise that there is plenty of free space, it is only within the Partition screen that there is no mention of it.) I came across the Terminal command to check the true amount of space on a drive, so I ran that just to check for inconsistencies. I can't seem to replicate it at the moment, but when I initially ran the command for disk0s2 it returned something like 'disk for disk not found', which I thought was weird. Before trying anything else in Terminal that I'd seen suggested as a potential fix, I decided to turn the computer off and on again— which leads me to where I am now:

Upon reopening Disk Utility after restarting the computer, I noticed that Macintosh HD had become 'greyed out' and was no longer mounted, indicating that the System Snapshot is mounted instead:

I am able to use and access everything seemingly as normal, but I have not been able to mount the drive. I have attempted to run First Aid and force-mount the drive using Terminal—both in recovery mode—but have not been able to change anything. This makes me worried.

I would like to figure out how to have Macintosh HD mounted, as normal, and then how to ensure that I can create the necessary partition to install Windows through Boot Camp.

I did run Windows in a partition on this machine a few years ago, which I may have removed or at least resized using Disk Utility rather than Boot Camp, so that might be worth noting?

Apologies if this was confusing to read and thanks in advance for any help.