Taking advice from further up the thread, I have now enabled “disable-synthetic-filelist” in my “Default options” and restarted one of the “bad” backup jobs.
Some good news, the backup is now running as it should, changing that option has definitely fixed the issue.
What is the drawback of disabling the synthetic file list? Does this mean that if I am to restore in the future I will not be able to use a partial backup (i.e. the previous incomplete backup that this stuck task was trying to resume) as a source?