Flashing the X96MAX back to X96Max_V311_4G_AP6356_1000M_20181116.img (same as the original android the box came with) with USB Burning Tool:
and the problem is back:
Flashing back to the newest firmware X96Max_V311_4G_AP6356_1000M_20190302.img
and the gigabit lan is working well again:
So obviously it was a software issue which has fixed in android.
We need the same fix in CE.