@Igor and who also have an interest in this issue/topic
After a long time - today Ethnernet is since hours stable again on my NanoPi Neo (no Network/eth0 down/up)
BUT I didnt changed the software (armbian)
Yesterday my NanoPi Neo had down/up (also) with Kernel 5.11.2-dev and debian buster and now today its stable since hours!
What I did? No it wasnt the software, nor the ethernet-cable, nor the Power-Supply, nor the eth0-setting.
I have a 16-port-Ethnernet-Hub with 100MBit, but its optical devided in 2x(2x4):
2x4 + 2x4 = 16 ports
The nanoPi Neo was in the 2nd 2x4 ports with the problem and after putting him in the 1st 2x4 port section the NanoPi Neo port did go stable.
In the 1st 2x4 ports are more or mostly (or only?) 100MBit devices and in the 2nd 2x4 are some/more 1GBit-devices.
I - personally - think the NanoPi Neo Ethernet-PHY didnt/doesnt tolerate some sort of ethernet-phy at the 2nd 2x4 port-part.
At the 1st 2x4port-part it seems he found ethernet-phys he could tolerate.
This is for today my "absurd" theory - BECAUSE it does work now for hours without a single up/down message in dmesg