Upload Speed being throttled from 200Mbps to 40Mbps

I have a 1.2Gbps plan. A few months ago I received an email asking me to upgrade my modem to increase the upload speed from 40Mbps to 200Mbps. So I bought a new modem (Hitron CODA56), and the uploaded speed is indeed increased.

But since a while ago (maybe a few weeks back) the upload speed is throttled to 40Mbps again. It's obviously due to throttling because in every speed test the upload can reach about 200Mbps in the first few seconds, and then it will drop to about 40Mbps only.

The test is performed on my x86 router which is directly connected to the modem. I've tried restarting the modem, renewing IP addresses. Nothing helped.

To Xfinity support: please help/explain.

Thanks.