diff --git a/test/ruby/enc/test_case_mapping.rb b/test/ruby/enc/test_case_mapping.rb
index dc14d8cb4f..d095cd569c 100644
--- a/test/ruby/enc/test_case_mapping.rb
+++ b/test/ruby/enc/test_case_mapping.rb
@@ -162,6 +162,31 @@ class TestCaseMappingPreliminary < Test::Unit::TestCase
     check_upcase_properties     'ΑΒΓΔΕΖΗΘΙΚΛΜΝΞΟΠΡΣΤΥΦΧΨΩ', 'αβγδεζηθικλμνξοπρστυφχψω'
   end
 
+  # This test checks against problems when changing the order of mapping results
+  # in some of the entries of the unfolding table (related to
+  # https://bugs.ruby-lang.org/issues/12990).
+  def test_reorder_unfold
+    # GREEK SMALL LETTER IOTA
+    assert_equal 0, "\u03B9" =~ /\u0345/i
+    assert_equal 0, "\u0345" =~ /\u03B9/i
+    assert_equal 0, "\u03B9" =~ /\u0399/i
+    assert_equal 0, "\u0399" =~ /\u03B9/i
+    assert_equal 0, "\u03B9" =~ /\u1fbe/i
+    assert_equal 0, "\u1fbe" =~ /\u03B9/i
+
+    # GREEK SMALL LETTER MU
+    assert_equal 0, "\u03BC" =~ /\u00B5/i
+    assert_equal 0, "\u00B5" =~ /\u03BC/i
+    assert_equal 0, "\u03BC" =~ /\u039C/i
+    assert_equal 0, "\u039C" =~ /\u03BC/i
+
+    # CYRILLIC SMALL LETTER MONOGRAPH UK
+    assert_equal 0, "\uA64B" =~ /\u1c88/i
+    assert_equal 0, "\u1c88" =~ /\uA64B/i
+    assert_equal 0, "\uA64B" =~ /\ua64A/i
+    assert_equal 0, "\ua64A" =~ /\uA64B/i
+  end
+
   def no_longer_a_test_buffer_allocations
     assert_equal 'TURKISH*ı'*10, ('I'*10).downcase(:turkic)
     assert_equal 'TURKISH*ı'*100, ('I'*100).downcase(:turkic)