Ethernet falls back from 1 Gbps to 100 Mbps randomly

I am on Windows 10.0.19045, 22H2. Motherboard has Realtek PCIe GbE Family Controller. Router is ASUS RT-AC66U. Both network interface and router are 1 Gbps capable. Connection is a wired ethernet cable. Periodically, the Realtek interface will fall back to 100 Mbps. What might cause this to occur? What, if anything, would prevent it from occurring?

Question by lit on Super User, licensed CC BY-SA. We show a short excerpt and link to the original thread as credit.