Thanks for the quick response!
I wasn’t using the same email address between the accounts, but I’ve changed it on Keycloak to match the one used in Discourse, and it’s still the same. The response in Data Explorer is the same (“NULL” user).
I tried creating a new, fresh user, and I’m getting a “Invalid username, email or password” error, even though the username is perfectly fine and available, the email is valid, and the password has been generated by my password manager. This is most likely unrelated but it’s something else I need to figure out why it’s happening.