Appling @Column(unique = true) doesn't lead to an unique constraint being created at database level.
I'd consider this bug critical since it fails silently and therefore leads to database corruption. The test data I used was unique therefore there should have been no reason for not applying the unique constraint
The issue happens on an already existing table, however, after doublechecking, the content of the database was containing only unique entries, therefore it should have been possible to apply the constraint.
Ubuntu LTS 18.4.2, Percona MySQL 8.0.15