CM500V reverts to default password?

I have recently gotten a CM500V cable modem with Comcast as my ISP. As usual, I reset modem the password to a new password and tested to make sure that I could log in using the new password. And then came back after a day or so and found that the modem password had reverted to the default. Did this 3 times, and the modem has reverted to the default Netgear password each time.

Wondering if this is a known problem? I don't know how or why my ISP would do this, but they may have a back door into cable modems so that they can flash their own firmware. But in the past they've changed my modem password to something other than the default, which makes sense. I really don't want anything on my network that's on the internet to have the default password. Just a little more vulnerable than I want to be.

So is this a defective modem, or could Comcast be doing this for some reason of their own? The modem works fine otherwise.