Thanks for your bug report. This is due to mIRC overriding the way desktop windows open to prevent them from taking focus from an existing window. This was added around 20 years ago and has been updated several times since then. It's a bit of a strange one because each time it has been updated, it has added more exceptions - clearly due to users not liking the behaviour in specific contexts. So I can add /dialogs to the exception list as well - although at this point, the list of exceptions is covering almost everything :-]
