Relay appears to work fine, but suddenly stopped receiving connections and appears as offline

I am apparently having a similar issue to this one described by this person on the tor-relays mail list

I have been running two non-exit relays for months without problems in a bare metal Debian machine. The relays are behind NAT and have dynamic IPs and the Tor version is (at time of writing), from the “official Tor Debian repo”.

All seemed to be running fine but suddenly (about 2 weeks ago) connections basically stopped coming through both relays and all my flags “dropped” in nyx (now shown as “none”). Short after, the relays appeared as “down” in metrics (at the time off writing they are not even in metrics anymore).

The strange thing is that nothing appears to be wrong:

  1. My IP and ports are reachable from the outside
  2. No errors or warnings in logs point to malfunction:
    2.1 In notice level logs I get the usual when restarting the service:
     External address seen and suggested by a directory authority: [PUBLIC IP]
     Now checking whether IPv4 ORPort [PUBLIC IP]:[ORPORT] is reachable... (this may take up to 20 minutes -- look for log messages indicating success)
     Self-testing indicates your ORPort [PUBLIC IP]:[ORPORT] is reachable from the outside. Excellent. Publishing server descriptor.
     Performing bandwidth self-test...done.

2.2 Changing log level to debug did not give any warnings/error pointing to malfunction
2.3 Using the MetricsPort also did not pointed to anything out of ordinary

After restarting any of the nodes they appear to make some connections (inbound, outbound, circuit, and directory according to nyx) for a short while, but after some time all connections drop with the exception of one inbound with the Authority dizum.

In the Consensus health website my relays are shown as getting votes from every authority (flags and bw) but the consensus column is empty.

Since nothing appeared to be wrong I just kept running the relays, but the situation did not return to normal and I decided to seek help here. Is there anyone experiencing something similar? How can I get any more information to diagnose this?