@Catshill Thanks for the comment. I've confirmed the suggested work around by deleting all of the text frames, re-pasting the text, and then setting the text style. This works. Interestingly, once I deleted all affected text frames, recreated the linked text frames, and applied the text styling, I could delete linked text frames to my hearts content, without any of the above text styling issues.
@walt.farrell Thanks for your suggestion. I've played with the resize frame option. The problem with that approach is that there is no easy way to match the font size, except for doing it by eye balling it. At least as far as I can see. Also, there is no IDML data involved in this case, so I'd guess that the problem is with common code used by IDML and native Affinity text style code.
@Catshill I confirmed Edit > Defaults > Revert works. Defaulting the text frame style, and then re-applying the custom style. This seems like the BEST workaround.
Cheers, Luke.