Ok then, this explains that.
If it can help, here are the json files, after the editor was closed, one of them in normal state (window won't reopen), and the other one in buggy state (window will reopen). (I was creating them in a clean environment, without any extension, while you was replying).
Edit : As KDE is widely used, maybe a way to prevent this bug would be to check, at __init__ of those windows, if they are maximized and if so, to unmaximized them ?
Edit two :
Argh, you're much faster than me! :-)
Thank you!