After saving all my tags, the files which have been tagged don't seem to have changed in their folder yet does display all the tags in the application. When the application is closed and reopened, the application reads all the 'tagless' files and shows all the tags are still there:
Files after being tagged:
The same files according to MP3Tag after closing and reopening application:
Not sure whether this is a bug but this issue hasn't happened in the past when I've done this and my process hasn't changed as far as I know.