unique constraint fails every second time

[fceller]

I've encountered a strange bug and have no idea, where it comes from:

AvocadoDB shell [V8 version 3.8.5, DB version 0.3.5]
avocado> db.examples.ensureUniqueConstraint("a")
{ "id" : 1194947, "unique" : true, "type" : "hash", "fields" : ["a"], "isNewlyCreated" : true }
avocado> x = db.examples.save({ a : 1, b : 1 })
80835/1653699
avocado> db.examples.document(x)
{ "_id" : "80835/1653699", "_rev" : 1653699, "a" : 1, "b" : 1 }
avocado> x = db.examples.save({ a : 1, b : 2 })
JavaScript exception in file '(avocado)' at 1,17: cannot save document: unique constraint violated
!x = db.examples.save({ a : 1, b : 2 })
! ^
avocado> db.examples.document(x)
{ "_id" : "80835/1653699", "_rev" : 1653699, "a" : 1, "b" : 1 }
avocado> db.examples.toArray()
[{ "_id" : "80835/1653699", "_rev" : 1653699, "a" : 1, "b" : 1 }]
avocado> x = db.examples.save({ a : 1, b : 2 })
80835/1981379
avocado> db.examples.toArray()
[{ "_id" : "80835/1653699", "_rev" : 1653699, "a" : 1, "b" : 1 }, { "_id" : "80835/1981379", "_rev" : 1981379, "a" : 1, "b" : 2 }]
avocado> bye...

The first save creates a document => fine
the second save gives a constraint violation => fine
the third save creates a document without violation => I've no idea why