If anyone else runs into a similar issue with the TUF-GAMING B550-PLUS motherboard, reinstalling AMD Adrenalin 2020 seemed to fix the problem (the build in question is using an AMD RX580 GPU). Uninstalled the AMD Adrenalin drivers from the Windows 10 control panel, then reinstalled Adrenalin 20.12.1 (latest as of this post) and opted to factory reset (within the Adrenalin installer) which seems to reset more than just the GPU to defaults.
After reinstalling AMD Adrenalin 20.12.1, restarting, and running some tests, the onboard Realtek PCIe 2.5GbE controller no longer seems to drop or have any problems. Wouldn't have expected that to fix the issue, but it did. I would've expected the BIOS to be the culprit, but only flash-back as a last resort, if reinstalling drivers doesn't correct the problems.