OClass.INDEX_TYPE.UNIQUE allows duplicates

[diaconescur]

OrientDB Version: Community edition 2.2.34 - cluster of 3 nodes;

Java Version: 1.8.0_172

OS: Windows 10

Expected behavior

We have multiple classes having a OType.STRING property as part of an unique index (OClass.INDEX_TYPE.UNIQUE). No duplicates should ever be inserted.

Actual behavior

We find in our db multiple records with the same value in this property.
The select:
select * from MyNodeClass where uniqueId='uniqueValue'
returns nothing;
The select by RID:
select from #153:693002
select from #155:713445
Brings both records, having the same string in the uniqueId.
An index rebuild obviously fails with:
Error: com.orientechnologies.orient.core.storage.ORecordDuplicatedException:
Cannot index record #153:693002: found duplicated key 'uniqueValue' in index 'MyNodeClass.uniqueId' previously assigned to the record #155:713445
DB name="myDb" INDEX=MyNodeClass.uniqueId RID=#155:713445

The node insert is performed in a transaction:
OrientGraphFactory factory = new OrientGraphFactory(...);
OrientGraph graph =factory.getTx();
factory.addVertex(...);
factory.commit();
factory.shutdown();

Steps to reproduce

Not clear how this happens. For weeks/months the application runs ok (normal behavior) and at some point we find this problems. Any idea what might cause this?