In addition to the previous method, I found another new way to reproduce this bug on my computer.
I right-click on the desktop item "Personalization" ➔ "Change the color of borders, windows and taskbar".
As soon as I move the sliders to change the color, then the mIRC client breaks down and starts displaying the same bug described posts above.
I'm not sure if it's just a simple lack of RAM. It seems to me that this bug occurs when is used on the computer external or system applications, that affect all system windows and begins to interact with MDI windows, affecting their appearance during general modification and reconfiguration. As you correctly noted "Photoshop" also has its own unique window design, and it probably during launch automatically reconfigures its windows to this unique style by interacting with the system Windows, which theoretically somehow affects other open applications, including MDI those used in the open mIRC client and breaks it.
It will probably be necessary to find some way to protect mIRC from such systemic impact that disfigures an open window in mIRC client and causes this bug.
I believe more that it has to do with some models of graphics cards because I followed the steps you indicated and I was unable to reproduce the bug.