Don't panic.
Sometimes it just happens. It could have been DLM doing a resync and for some reason the line just didnt come back up properly. Happens to the best of routers. Could just have easily happened with the HH5. Only difference is you wouldn't have been able to monitor and wouldn't have known till you got home.
it means DLM will not increase the Downstream Rate as quickly as hoped.
Seriously. Stop. Listen.
One extra resync isnt going to bother the DLM. Also, because it was off for several hours it will be seen as an unforced retrain and not counted!
I think you may also be getting in a tizz re the FEC. I note that its not unusual for you to have burst FECs nearing 30k. DLM monitors Err/Secs. FEC just shows that interleaving is working as it should.
Please, please, please dont get too worked up over the little things. Your traffic lights are still all green so all is good.
TBH one of the reasons why some ISPs use routers that dont show all stats is because some EU's used to get worked up about things which were perfectly normal.