A few month ago I had similar issues with the GBit-Ethernet-Adapter in my T420 Thinkpad. Then it didn't activated the interface after going to sleep and waking up again. A workaround was to use ethtool to do a quick test, which also woke the ethernet interface up again. Later I had issues again and could solve them by switching from NetworkManager to systemd-networkd. Now, with Kernel 5.0.x the Interface takes about 30-60s to wake up, after the system has been woken. This alone is annoying, but the second thing is, that auto-negotiation only set the interface to 100 Mbit/s. If I unplug and replug the Ethernet it will go back to 1000 Mbit/s.
Somehow Manjaro doesn't like the ethernet interfaces on this machine. Never had any issues with Ubuntu or Windows before. I tested the cable
(Cat6a) and switch port (Netgear ProSafe) - they seem to be perfectly fine and also worked stable for years.