After some troubleshooting the fault appears to be with the Adtran ONT, which I managed to clear with a turning-it-off-and-on.
Before the reboot I tried connecting my laptop directly with a 10GbE adapter to the Adtran ONT, the speedtest there was also faulty - 3Gbps down, around 1Gbps up consistent.
As a last ditch attempt to troubleshoot this, I unplugged the power to the Adtran ONT and plugged it back in.
To my surprise, as soon as it came back up, my internet speed was back to 3Gbps/3Gbps. I then swapped the connection back to my router (CCR2004), and all was back to the original 3Gbps/3Gbps speed.
No idea if someone was toying with connection profiles and left my ONT using the wrong speed, but this wasn't great since I would have no way of resetting it if I'm away from home.
I've raised a ticket with CFL (R-186643412) hopefully someone from CFL will know enough to pull some logs and see what happened there.