diff --git a/stl/inc/xlocnum b/stl/inc/xlocnum
index afe27df46d..cb0ddec757 100644
--- a/stl/inc/xlocnum
+++ b/stl/inc/xlocnum
@@ -1371,10 +1371,13 @@ protected:
         }
 
         _Buf.resize(_Bufsize + 50); // add fudge factor
+        const bool _Is_finite      = (_STD isfinite)(_Val);
+        const auto _Adjusted_flags = // TRANSITION, DevCom-10519861
+            _Is_finite ? _Iosbase.flags() : _Iosbase.flags() & ~ios_base::showpoint;
         const auto _Ngen = static_cast<size_t>(_CSTD sprintf_s(
-            &_Buf[0], _Buf.size(), _Ffmt(_Fmt, 0, _Iosbase.flags()), static_cast<int>(_Precision), _Val));
+            &_Buf[0], _Buf.size(), _Ffmt(_Fmt, 0, _Adjusted_flags), static_cast<int>(_Precision), _Val));
 
-        return _Fput_v3(_Dest, _Iosbase, _Fill, _Buf.c_str(), _Ngen, (_STD isfinite)(_Val));
+        return _Fput_v3(_Dest, _Iosbase, _Fill, _Buf.c_str(), _Ngen, _Is_finite);
     }
 
     virtual _OutIt __CLR_OR_THIS_CALL do_put(
@@ -1395,10 +1398,13 @@ protected:
         }
 
         _Buf.resize(_Bufsize + 50); // add fudge factor
+        const bool _Is_finite      = (_STD isfinite)(_Val);
+        const auto _Adjusted_flags = // TRANSITION, DevCom-10519861
+            _Is_finite ? _Iosbase.flags() : _Iosbase.flags() & ~ios_base::showpoint;
         const auto _Ngen = static_cast<size_t>(_CSTD sprintf_s(
-            &_Buf[0], _Buf.size(), _Ffmt(_Fmt, 'L', _Iosbase.flags()), static_cast<int>(_Precision), _Val));
+            &_Buf[0], _Buf.size(), _Ffmt(_Fmt, 'L', _Adjusted_flags), static_cast<int>(_Precision), _Val));
 
-        return _Fput_v3(_Dest, _Iosbase, _Fill, _Buf.c_str(), _Ngen, (_STD isfinite)(_Val));
+        return _Fput_v3(_Dest, _Iosbase, _Fill, _Buf.c_str(), _Ngen, _Is_finite);
     }
 #pragma warning(pop)
 
diff --git a/tests/std/tests/GH_003867_output_nan/test.cpp b/tests/std/tests/GH_003867_output_nan/test.cpp
index e36c6779fc..059ed13cca 100644
--- a/tests/std/tests/GH_003867_output_nan/test.cpp
+++ b/tests/std/tests/GH_003867_output_nan/test.cpp
@@ -2,6 +2,7 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
 #include <cassert>
+#include <iomanip>
 #include <limits>
 #include <locale>
 #include <sstream>
@@ -54,8 +55,56 @@ void test_gh_3867() {
 #endif // TEST_CUSTOM_FACET
 }
 
+// Also test GH-4210: With setprecision(0) showpoint fixed, a bogus '.' is emitted for infinity and NaN
+
+template <class FloatingPoint>
+void test_output_nonfinite_value(const FloatingPoint x) {
+    const auto s1 = [x] {
+        ostringstream os;
+        os << setprecision(0) << showpoint << fixed;
+        os << x;
+        return os.str();
+    }();
+    const auto s2 = [x] {
+        ostringstream os;
+        os << setprecision(0) << noshowpoint << fixed;
+        os << x;
+        return os.str();
+    }();
+    assert(s1 == s2);
+
+    const auto s3 = [x] {
+        ostringstream os;
+        os << setprecision(0) << showpoint << fixed << showpos;
+        os << x;
+        return os.str();
+    }();
+    const auto s4 = [x] {
+        ostringstream os;
+        os << setprecision(0) << noshowpoint << fixed << showpos;
+        os << x;
+        return os.str();
+    }();
+    assert(s3 == s4);
+}
+
+template <class FloatingPoint>
+void test_gh_4210() {
+    constexpr auto inf_val = numeric_limits<FloatingPoint>::infinity();
+    constexpr auto nan_val = numeric_limits<FloatingPoint>::quiet_NaN();
+
+    test_output_nonfinite_value(inf_val);
+    test_output_nonfinite_value(-inf_val);
+    test_output_nonfinite_value(nan_val);
+    test_output_nonfinite_value(-nan_val);
+}
+
 int main() {
     test_gh_3867<float>();
     test_gh_3867<double>();
     test_gh_3867<long double>();
+
+    test_gh_4210<float>();
+    test_gh_4210<double>();
+    test_gh_4210<long double>();
 }