I’ve investigated the issue.
The trust level is indeed set to 0, even in the database, but it strangely doesn’t affect the user in any way. It continues to see and use the forum accordingly to its original trust level.
As soon as the user does an action (opening a topic, posting a message…), the trust level returns to its initial value in the database and the interface.
It’s unlikely to cause any issues, but I’ve reported the bug to the team nonetheless.