D'oh, seems I maybe spoke too soon! In the editor, when I play the game, it seems fine, but as soon as I try and record with Unity's recorder, I can see the quick movement before the text gets moved into place. I've tried your other suggestions, and wondered if I could please try that script you suggested that helps with the ContentSizeFitter?
Re your other question, on the object that has Super Text Mesh on for subtitles, I also have a 'Text Mesh Pro' object on it too that's hidden as it's the only way I can get Adventure Creator's menu to accept the 'linked text' object, as it's looking for a TMP component.
Example of what's happening with recorder: