remapping - Autohotkey Ctrl+Shift - Zbrush/Photoshop

I have been using Autohotkey for a long time to remap keys inside CG softwares (mainly Zbrush and Photoshop) to make them more lefty-friendly.

Essentially, I remap the Numpad keys to keys you can't change inside the programs (Ctrl, Alt, Shift, etc...). This way I can use my graphics tablet pen with my left hand and access Ctrl, Alt, etc. using the Numpad, which is much more convenient than offsetting the whole keyboard to the right.

Anyway, here is my script :

 #IfWinActive, ahk_class ZBrush
 #MaxHotkeysPerInterval 200
 NumPad1::Alt
 Numpad2::LControl
 Right::Shift
 up::Space
 return

Pretty simple, but I have had a bug with it ever since I created it several years ago. Essentially, when I am pressing and holding Numpad2+Right (Ctrl+Shift), and release Numpad2, the program behaves as if I was still pressing it. Repressing Numpad2 real quick sets it back to normal but it's very annoying. Here are a couple of images to better illustrate the issue :

Do you know why this happens? I am no programmer, so I could definitely use some help.