From the GitHub forums:

"What is expected when you use 2 not operators in sequence like User.not.not.where(field: true)? Shouldn't the query User.where(foo: true) be equivalent to User.not.not.where(foo:true)? I am finding that this is not the case, and am wondering if this usage of the not operator is unsupported or if this is a bug?"

I've confirmed this is a bug – specifying not a second time does not toggle the criteria's negation state as it should.

See: https://github.com/mongodb/mongoid/blob/master/lib/mongoid/criteria/queryable/selectable.rb#L556