Wavelength Service - Protocols down, but link state up?

Hey All,

I used to see this a lot when I worked for an MSP, our datacentre backbone links would sometimes lose ISIS/BFD sessions on the link but the actual physical connection would remain up during a planned outage. I'm wondering what causes this behaviour?

I work for a normal ISP now and one of our backhauls experienced this, I can't see any issue on our devices so I've reached out to the link provider to see if it was an unplanned maintenance. We did have an issue on our devices with our overlay BFD/BGP session going down due to a large number of ARP DDoS protocol violations but we can't see the same issue on the switch at the moment so I'm assuming for now it was possibly the fibre...