I've moved my Hybrid Connect close to my Smart Hub 2 so that it can connect via Ethernet. I did have the HC upstairs to get a better mobile signal but it kept losing its connection to the SH2, as evidenced by the central light flashing red. I had also observed the HC would often display flashing blue. I understand (from BT's help page) that this means the HC "is pairing with the hub".
Now the HC is connected to the SH2 over Ethernet, it still displays flashing blue, and quite frequently. I haven't measured or counted, but it happens several times per hour.
I'm puzzled why the HC should be "pairing with the hub" when it displays solid blue between these episodes - which means (again according to BT's webpage) that the "device is paired with your hub."