Regression on elemMatch?

Here's a code that works on mongoengine version 0.9.0, but raises exception on 0.10.6:

import mongoengine as mongo


class Top(mongo.Document):
    class Parent(mongo.EmbeddedDocument):
        class Child(mongo.EmbeddedDocument):
            somedata = mongo.IntField()

        value = mongo.DynamicField()  # <-- can be of Child type or untyped (without _cls)

    plist = mongo.ListField(field=mongo.EmbeddedDocumentField(Parent))


def method1():
    # Method 1
    print 'ComboQ:'
    untypedQ = mongo.Q(__raw__={'value': {'_cls': {'$exists': False}}}) & mongo.Q(value__lte=-1)
    childQ = mongo.Q(value=Top.Parent.Child()) & mongo.Q(value__somedata__gte=1)
    comboQ = untypedQ | childQ
    print comboQ.to_query(Top.Parent)

    print 'TopQ:'
    topQ = mongo.Q(plist__match=comboQ.to_query(Top.Parent))
    print topQ.to_query(Top)


def method2():
    # Method 2
    print 'ComboQ:'
    untypedQ = mongo.Q(__raw__={'value': {'_cls': {'$exists': False}, 'lte': -1}})
    childQ = mongo.Q(__raw__={'value': {'_cls': 'Child', 'somedata': {'gte': 1}}})
    comboQ = untypedQ | childQ
    print comboQ.to_query(Top.Parent)

    print 'TopQ:'
    topQ = mongo.Q(plist__match=comboQ.to_query(Top.Parent))
    print topQ.to_query(Top)


if __name__ == '__main__':
    for method in [method1, method2]:
        try:
            method()
        except Exception as e:
            print u'{}: {}'.format(e.__class__.__name__, str(e))
        print

Output on version 0.9.0:

ComboQ:
{'$or': [{'value': {'_cls': {'$exists': False}, '$lte': -1}}, {'value.somedata': {'$gte': 1}, 'value': SON([('_cls', 'Child')])}]}
TopQ:
{'plist': {'$elemMatch': {'$or': [{'value': {'_cls': {'$exists': False}, '$lte': -1}}, {'value.somedata': {'$gte': 1}, 'value': SON([('_cls', 'Child')])}]}}}

ComboQ:
{'$or': [{'value': {'_cls': {'$exists': False}, 'lte': -1}}, {'value': {'somedata': {'gte': 1}, '_cls': 'Child'}}]}
TopQ:
{'plist': {'$elemMatch': {'$or': [{'value': {'_cls': {'$exists': False}, 'lte': -1}}, {'value': {'somedata': {'gte': 1}, '_cls': 'Child'}}]}}}

Output on version 0.10.6:

ComboQ:
{'$or': [{'value': {'_cls': {'$exists': False}, '$lte': -1}}, {'value.somedata': {'$gte': 1}, 'value': SON([('_cls', 'Child')])}]}
TopQ:
InvalidQueryError: Cannot resolve field "$or"

ComboQ:
{'$or': [{'value': {'_cls': {'$exists': False}, 'lte': -1}}, {'value': {'somedata': {'gte': 1}, '_cls': 'Child'}}]}
TopQ:
InvalidQueryError: Cannot resolve field "$or"

Is it regression or some intended design?

Added:
Output on version 0.10.0:

ComboQ:
{'$or': [{'value._cls': {'$exists': False}, 'value': {'$lte': -1}}, {'value.somedata': {'$gte': 1}, 'value': SON([('_cls', 'Child')])}]}
TopQ:
FieldDoesNotExist: The field '$or' does not exist on the document 'Parent'

ComboQ:
{'$or': [{'value._cls': {'$exists': False}, 'lte': -1}, {'value._cls': 'Child', 'somedata': {'gte': 1}}]}
TopQ:
FieldDoesNotExist: The field '$or' does not exist on the document 'Parent'

Which means regression occured between versions 0.10.0 and 0.9.0

Run the supplied reproduction against MongoEngine 0.9.0, 0.10.0, and 0.10.6, focusing on Q.to_query with an $or expression nested in __match. Trace the query transformation that produces the '$or' field-resolution error, and consider the issue done when both methods generate the working elemMatch query shown for 0.9.0 without raising an exception.