Thanks! I have to RMA my device due to a hardware issue, so not sure if I get a hand on testing before I send it to the distributor - so I'm glad you confirmed.I can confirm that this happens on 6.41rc11
Putting off power from the device for an hour seems to have helped, having a sync with 1 Gbps again now.I more and more think it's hardware-related.
I just put a switch in between the heX and the cable modem, and voilà, full sync with 1 Gbps
> /system routerboard print routerboard: yes model: RouterBOARD 750G r3 serial-number: ### firmware-type: mt7621L factory-firmware: 3.34 current-firmware: 3.35 upgrade-firmware: 3.35
/ip dns set allow-remote-requests=yes servers=2001:4860:4860::8888,2001:4860:4860::8844,188.8.131.52,184.108.40.206