What version of Central are you using? To see your version, click on the question mark icon in the upper right section of your Central menu bar, then click Version. If you don't see the question mark, you can see the version by adding version.txt to the root URL (e.g., https://demo.getodk.cloud/version.txt).
What happens if you run this command? Does it print a 1, a 0, or nothing?
docker exec -it central-enketo-1 curl https://$(grep "^DOMAIN=" ~/central/.env | cut -d = -f 2) | grep -c getodk.org