Minitest 5.6.0 breaks tests with JSON/XML responses mocked with OpenStruct

Yesterday my CI system started a-smokin' and a-screamin' and many of the errors were something along these lines:

--- expected
+++ actual
@@ -1 +1 @@
-"<chart><series name=\"Lineups\"><name>56</name><name>614</name><name>0</name></series></chart>"
+"<chart><series name=\"Lineups\"><name>#&lt;struct Minitest::Expectation target=nil, ctx=#&lt;OpenStruct name=\"match day #abc\", value=56&gt;&gt;</name><name>#&lt;struct Minitest::Expectation target=nil, ctx=#&lt;OpenStruct name=\"match day #def\", value=614&gt;&gt;</name><name>#&lt;struct Minitest::Expectation target=nil, ctx=#&lt;OpenStruct name=\"match day #012\", value=0&gt;&gt;</name></series></chart>"

Note that the expected values have been replaced with Minitest::Expectation target=nil. The test for this case is building some sample input:

def build_sample_input
  [
    OpenStruct.new(
      :name   => "match day #abc",
      :value  => 56
    ),
    ...
  ]
end

and the code that's being tested uses the input with Nokogiri and builds some XML output.

builder = Nokogiri::XML::Builder.new do |xml|
  process_source(xml)
end

def process_source(xml)
  xml.chart do
    xml.show_graph_ false
    xml.type_ 'line'
    xml.orientation_ 'vertical'
    xml.description_ 'Unique Lineup Selections'
    xml.categories do
     input.each do |match_day|
        xml.name_ match_day.name
     end
    end
    xml.series('name' => 'Lineups') do
      input.each do |match_day|
        xml.name_ match_day.value
      end
    end
  end
end

Anyone else run into this? I know it's pretty specific. Bug report is here