repos.key missing the release key?

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I'm following the directions here to install ROS2 Crystal on Ubuntu 18.04. I've just done the curl http://repo.ros2.org/repos.key | sudo apt-key add - step, but when I follow that up with apt-get update, I get a warning that the signatures couldn't be verified (NO_PUBKEY 5523BAEEB01FA116). Sure enough, that key is not downloaded in the curl step, according to the output of apt-key list. This has definitely worked for me in the past - did the key change or something like that?