diff --git a/tests/uchar_test.cpp b/tests/uchar_test.cpp
index d7174bd..0114adf 100644
--- a/tests/uchar_test.cpp
+++ b/tests/uchar_test.cpp
@@ -182,11 +182,40 @@
   ASSERT_EQ(3U, mbrtoc16(&out, "\xe2\x82\xac" "def", 6, nullptr));
   ASSERT_EQ(static_cast<char16_t>(0x20ac), out);
   // 4-byte UTF-8 will be returned as a surrogate pair...
+#ifdef __BIONIC__
+  // https://issuetracker.google.com/289419882
+  //
+  // We misread the spec when implementing this. The first call should return
+  // the length of the decoded character, and the second call should return -3
+  // to indicate that the output is a continuation of the character decoded by
+  // the first call.
+  //
+  // C23 7.30.1.3.4:
+  //
+  //     between 1 and n inclusive if the next n or fewer bytes complete a valid
+  //     multibyte character (which is the value stored); the value returned is
+  //     the number of bytes that complete the multibyte character.
+  //
+  //     (size_t)(-3) if the next character resulting from a previous call has
+  //     been stored (no bytes from the input have been consumed by this call).
+  //
+  // Leaving the test for the wrong outputs here while we clean up and improve
+  // the rest of the tests to get a better handle on the behavior differences
+  // before fixing the bug.
   ASSERT_EQ(static_cast<size_t>(-3),
             mbrtoc16(&out, "\xf4\x8a\xaf\x8d", 6, nullptr));
   ASSERT_EQ(static_cast<char16_t>(0xdbea), out);
   ASSERT_EQ(4U, mbrtoc16(&out, "\xf4\x8a\xaf\x8d" "ef", 6, nullptr));
   ASSERT_EQ(static_cast<char16_t>(0xdfcd), out);
+#else
+  ASSERT_EQ(4U, mbrtoc16(&out, "\xf4\x8a\xaf\x8d", 6, nullptr));
+  ASSERT_EQ(static_cast<char16_t>(0xdbea), out);
+  ASSERT_EQ(static_cast<size_t>(-3), mbrtoc16(&out,
+                                              "\xf4\x8a\xaf\x8d"
+                                              "ef",
+                                              6, nullptr));
+  ASSERT_EQ(static_cast<char16_t>(0xdfcd), out);
+#endif
 }
 
 TEST(uchar, mbrtoc16_long_sequences) {
@@ -208,9 +237,13 @@
 }
 
 TEST(uchar, mbrtoc16_reserved_range) {
-  char16_t out;
-  ASSERT_EQ(static_cast<size_t>(-1),
-            mbrtoc16(&out, "\xf0\x80\xbf\xbf", 6, nullptr));
+  ASSERT_STREQ("C.UTF-8", setlocale(LC_CTYPE, "C.UTF-8"));
+
+  errno = 0;
+  char16_t out = u'\0';
+  EXPECT_EQ(static_cast<size_t>(-1), mbrtoc16(&out, "\xf0\x80\xbf\xbf", 6, nullptr));
+  EXPECT_EQ(u'\0', out);
+  EXPECT_EQ(EILSEQ, errno);
 }
 
 TEST(uchar, mbrtoc16_beyond_range) {
@@ -238,10 +271,25 @@
   // 4-byte UTF-8.
   ASSERT_EQ(static_cast<size_t>(-2), mbrtoc16(&out, "\xf4", 1, ps));
   ASSERT_EQ(static_cast<size_t>(-2), mbrtoc16(&out, "\x8a\xaf", 2, ps));
+#ifdef __BIONIC__
+  // https://issuetracker.google.com/289419882
+  // See explanation in mbrtoc16 test for the same bug.
   ASSERT_EQ(static_cast<size_t>(-3), mbrtoc16(&out, "\x8d" "ef", 3, ps));
   ASSERT_EQ(static_cast<char16_t>(0xdbea), out);
   ASSERT_EQ(1U, mbrtoc16(&out, "\x80" "ef", 3, ps));
   ASSERT_EQ(static_cast<char16_t>(0xdfcd), out);
+#else
+  ASSERT_EQ(1U, mbrtoc16(&out,
+                         "\x8d"
+                         "ef",
+                         3, ps));
+  ASSERT_EQ(static_cast<char16_t>(0xdbea), out);
+  ASSERT_EQ(static_cast<size_t>(-3), mbrtoc16(&out,
+                                              "\x80"
+                                              "ef",
+                                              3, ps));
+  ASSERT_EQ(static_cast<char16_t>(0xdfcd), out);
+#endif
   ASSERT_TRUE(mbsinit(ps));
 
   // Invalid 2-byte