OK, I see what's going on. Note that mIRC is writing to the file correctly, but mIRC does not add a
byte order mark (BOM) to the beginning of the file. If no BOM is present, a text editor has to guess whether or not the file is Unicode or plain ASCII. In this case, Wordpad appears to be assuming that all text is plain ASCII unless it sees a BOM, while Notepad will use intelligent "guessing" if no BOM is present.
Depending on your application, you may or may not want a BOM in the file so I don't think it is a good idea for /write to automatically add it. If this is only a one-time issue where you need to add a BOM to the file, an easy way to do it is to just open the file in Notepad then save it, as Notepad will automatically add the BOM if it's not already present.