unexpected mutability.rb

def combine_query(query, piece)
    # this function always receives its previous return value
    _query = {}.merge(query) # also tried Hash.new(), query.dup, query.clone
    _query['query'] ||= {}
    _query['query']['bool'] ||= {}
    _query['query']['bool']['must'] ||= []
    _query['query']['bool']['must'] << piece
    _query
end

def aggregate(by: {}, query: {}, &block)
    results = search_index(body: query.dup.merge({aggs: get_aggregations(by)}))
    results.each do |result|
        display, value = result['key'].split('|')
        yield value, display
    end
end

def main_job_perform_function
    # [ .... within a loop ....]
    q = get_desided_query.freeze
    # q is the object that is being unexpectedly mutated
    aggregate by: {field: 'my_field'}, query: q do |k, v|
        new_query = combine_query(q, get_other_condition('my_field', k))
        update_index(query: new_query['query'], script: populate_array_field({ myfield: myvalue }))
        new_query = 'ASDASAd'
        log_debug(new_query)
    end
    # [ .... ]
end