As a further update, I have opened a Kernel bug ( https://bugzilla.kernel.org/show_bug.cgi?id=213481 ), as this appears to be a bug introduced with commit 4514d991d99211f225d83b7e640285f29f0755d0 ( https://github.com/torvalds/linux/commit/4514d991d99211f225d83b7e640285f29f0755d0 ) to the kernel which, in the 5.10.x branch was introduced in 5.10.36.
It would appear something about this change that the e1000e driver is not handling properly on this system, causing the card to not work.
Hopeful for a fix and may be wise to incorporate into the current kernel before the final release of Debian 11, as this will likely break other users' systems on the upgrade from Debian 10 to 11.