I have the same problem with CCR2116-12G-4S+ router.
The problem happens once in a while, can be an hour, 5 days, 2 months, super random.
When this happens, part of the router’s functionality becomes unavailable, the router’s fan makes a sound like it stops and starts repeatedly.
I confirm that I use 2 sfp onu from FS.
The problem is not solved until I unplug the router from the power and plug it back in…
I encountered exactly the same problem with a CRS305-1G-4S+IN, I had to enter manually to deactivate and reactivate the RouterOs interface, after that it worked.
I have the same issue and I’m trying to get a fix as the router fans spin up and down and the router hangs a lot when trying some modifications.
What I don’t understand is if it works with another router, it should also work with this one or is there some modifications in the gpon (or disable something?)