Duplex and speed mismatches are quite common on Ethernet speed auto-negotiate , especially at Gigabit speeds.
Duplex mismatch
Plenty of other references on a Google search.
It only needs a slight timing mismatch between the ONT and the Smart Hub, for the speed to fall back to 100Mbs.
As both devices are mass produced at minimum cost, it quite likely that a mismatch could occur. Even swapping to another Smart Hub 2, might make a difference to the timing.
Unless there is a facility on either device, to lock the Ethernet speed to 1000Mbs Full Duplex, then this problem is likely to occur.
Sometimes putting a 1Gb Ethernet switch between the two devices, can fix the timing issue, and make both negotiate the correct speed.
Other third party routers may present a better physical connection speed, as some forum members have reported better results.
