Thanks for your bug report. I have not been able to reproduce this issue so far. Can you please provide a step by step method that reproduces this issue, including a minimal script that creates a custom dialog that does not minimize for you?
Thank, I was able to reproduce it. It looks like you have found a Windows bug. If you do not use the "notheme" option, the minimize button works. If you tell Windows to disable themes by using "notheme", Windows fails to handle the mimimize button correctly.
I just tried /dialog -i test on the above dialog and it minimized it. Can you please test this again and confirm? That said, the "notheme" option is a simple call to Windows to disable theming on a Window. There is no other method of enabling/disabling theming. So if Windows is not handling it correctly, there is nothing mIRC can do to resolve it.