command line - How to make NppExec show script-created windows?

I am trying to run a Python script with NppExec. It runs fine (the console is updated with whatever I print to stdout in the script) but the window the script creates (through PyQt) never shows up. If I execute the same script in a standard shell (cmd.exe), the window shows up just fine. Actually, the splash screen of the application I want to launch (vitables) shows up under NppExec but not the main window.

Update: Strangely, I noticed it works fine on some machines (but not on others). Both the machines where it works and those where it does not use the same OS (Windows 7 SP1), the same Notepad++ version (5.8.6) and NppExec version (0.4.1). I have tried updating to the latest versions (npp 6.5.3 and nppExec 0.5) but it does not fix the problem.