is_not_trusted = 1 after Error-Free WITH CHECK CHECK CONSTRAINT
Thanks to a How to make foreign key constraints trusted on StackOverflow, I figured out that the constraint was untrusted because it was marked NOT FOR REPLICATION. After re-creating the constraint without NOT FOR REPLICATION, all were trusted!

This behavior makes sense. If the constraint isn't replicated, other replication participants won't be enforcing it. When changes made by those other participants make their way back to the database I'm working with, there's no guarantee that the foreign key constraint's condition is satisfied—so NOT FOR REPLICATION forces the constraint to be marked is_not_trusted.