IntelliJ's built-in SSH client seems to hash its known_hosts, but the one I had had its host names in clear text.
When I deleted the file and let IntelliJ create a new one, with only my (hashed) GitLab server and nothing else, it worked.
It's also not possible to mix it - keep some unhashed entries together with hashed entries for IntelliJ. So, you have to configure your other SSH clients to use hashed hosts.