i went through something similar (got another card as a result of my support/RMA ticket, new card had the same issue)
i was able to find this post from another thread that helped me: [SOLVED] Ethernet expansion card is not connecting - #11 by anarcat
essentially, it seems like the power management (tlp i think) on ubuntu is doing something incorrectly with this ethernet card. i see the same behavior on both of my basically-new ethernet cards.
the linked post has a command (something like echo 'on' > '/sys/bus/usb/devices/2-3/power/control'; ) to make sure the card is on, not being auto-power-managed by tlp. This mayde my ethernet cards work as expected. Farther down in the thread theres also instructions on how to exclude the eternet cards from TLP more permanently in case you dont want to run a command like that every time.
although this is probably more of a workaround, so it may still be worth following up with support to help them find the root cause of why tlp isnt playing nice with these cards.
it also looks like TLP is open source, so maybe, if this is reproducible on other ethernet devices with a RTL8156 controller, it might partly be solveable with a contribution to TLP to better handle that controller.
FWIW, i have a UGREEN brand USB-C dongle with an RTL8153 in it (gigabit) and that has always worked fine. not sure how similar those controllers are tho. there do seem to be a few results for RTL8153 in the TLP repo and none for 8156. maybe that says/points to something useful?