The issue is well described in https://github.com/joomla/joomla-cms/issues/34368
The proposed corection in this github issue is
Thanks @venomdev! As I have access to the template I ended up solving it in a different way. Apologies that this is not really the place for this discussion but for anyone looking, you can add a line to your template to run the onContentPrepare plugins on the introtext and/or fulltext properties. For example: $intro_prepared = JHtml::_('content.prepare', $this->item->introtext); echo $intro_prepared;
If it's just the fulltext you need to prepare, set the global content parameters to hide the intro, then you can use $this->item->introtext and $this->item->text separately in your template.
Would it be possible to make such a change in Purity IV & Simpli ?