The PixInsight core application does not use the Windows registry at all. On Windows, file associations with the PixInsight core application must be established manually by the user. This limitation is by design, so there is no bug. For some reason, the file associations that you had created for version 1.8.3 have not been preserved when you have installed version 1.8.4, so you'll have to create them again.
On OS X and modern X11 desktops (Linux/FreeBSD) file associations are managed automatically by the operating system in a clean and safe way through configuration files (.plist and .desktop files, respectively). The Windows registry is an obsolete, obscure and unsafe structure IMO, which has caused us lots of problems and compatibility issues in the past. Perhaps a future version of PixInsight for Windows will use it somewhat, but right now we are well without depending on it, so this is not a priority.