Native c++ (mingw) plugin works in editor but not in build

I have a minimal c++ script that's called from a minimal c# script. The dll is compiled on Windows 10 using mingw g++ and it works fine in the editor. Build an run however leads to the following error message:

Plugins: Failed to load 'C:/(...)/dll-example_Data/Plugins/libTest.dll' with error 'A dynamic link library (DLL) initialization routine failed.

Here's the .cs file:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using System.Runtime.InteropServices;
 
 public class TestScript : MonoBehaviour
 {
     [DllImport("libTest", CallingConvention = CallingConvention.StdCall, ExactSpelling = true)]
     public static extern int TestFunction();
 
     void Start()
     {
         Debug.LogFormat("libTest says: {0}", TestFunction());
     }
 }

And the .cpp:

 extern "C" {
 int __stdcall __declspec(dllexport) TestFunction() {
         int* test = new int[1]; // removing this line will make everything work when building
         return 4;
 }
 }

Here's how I build it:

 g++ -O2 -g -Wall -c -fmessage-length=0 Test.cpp
 g++ -shared -o libTest.dll Test.o

So If I delete the marked line (int* test = ...), recompile and replace the .dll everything works fine using the build .exe and the editor.

Any suggestions would be appreciated!

I've tried finding a difference between the dlls using ldd, objdump and dumpbin.exe but couldn't find anything obvious, however I'm not quite an expert with these tools.