Hi. I am actually experiencing a very similar issue.
I have a gigabit plan and use the provided XB6 modem in bridge mode.
My edge device for a while was a system I had running Pfsense. I had no issusues with speed for over six months. I had regularly performed speed tests due to some projects I had been working on. I recently switched over to a Ubiquity edge router 4 due to the way Pfsense does NAT. I have quite a few roomates who like to play together in the same online games. Anyways, when I setup the edge router, I of course did a speed test. I was only getting 560Mb/s max. I thought that this was due to the config on the new router, so I connected my PC directly to the modem. and rebooted it. I was still only getting around 500 Mb/s. I contacted comcast, and over the past month I have had 4 techs out to my house. They have tested the lines and replaced my original modem.
after all the testing and troubleshooting we have done, the only time I am able to get my full gigabit speeds are when I have the modem reprovisioned and dont put it in bridge mode. As soon as I put it in bridge mode, I can only get around 500 Mb/s. Taking it back out of bridge mode does nothing.
It has been suggested that we try to get one of the new XB7 modems and see if it solves anything.
It was also recommended that we just downgrade our package to the 500Mb/s plan.
This issue has lasted for a month so far. I was able to get my bill cut in half, but it took a lot of back and forth. We are going to go up to one of the comcast stores and see if they can provovide us with one of the new model modems.
AFAIK, the XB7 is not generally available at this time.
Did you read this?
I am having this same issue. I spoke with Xfinity and attached the chat. Apparently there is nothing they said they can do about it. This is a joke.