Custom Email post-function is reverting (and not saving updates) from HTML to Plain text

We recently upgraded ScriptRunner to v5.6.8 and have noticed that all of our existing 'Custom email' post functions have reverted to 'Plain text' email format when they were originally 'HTML' format. When we try to edit the post function to change it to 'HTML', after saving and re-opening it reverts back to 'Plain text'