I have moved on. The description here is vague and probably doesn't make any sense.

Potentially relevant bug on debian's bug tracker

This has happened a few more times since last report.

This happened again.

oh okay. I don't know what else to do. this is difficult to replicate and it's scary if this happens when i am not at home and primary wan is also unavailable.

This has happened 2-3 times since my last comment. I'll keep this open until kea-dhcp is merged and then check if this bug remains.

I believe it's probably generating this error because modem sends an empty hostname.

Logs from just dhclient.

I didn't update it since the initial report and I don't really understand how/why it's working again.

I would still recommend you to try to test to put a L2-switch between your 5G-router and the VyOS box and see if that resolves the situation.

Does your 5G-modem do any NAT on its own or does it just forward the DHCP to the ISP?

Could also be if legacy spanning-tree is being used that it will take approx 25 seconds before packets are being forwarded after a linkdown - workaround here is to enable "spanning-tree portfast" or disable spanning-tree.