Hi @fletsch
thank you for trying FDP.
The preview https://nimbusweb.me/nimbus-screenshots/a88852e5724563e5276e56918191e712 is related to the plugins that you can disable in Singles https://nimbusweb.me/nimbus-screenshots/7d7fe090e9924b227dbb0a47d6394a78 also if you do not save the Singles settings.
If you disable some plugins in Post Types and save the Post Types settings, those plugins are disabled when you visit a single post. The preview is something else. The preview has nothing to do with the Post Types settings.
Usually, you use the preview in SIngles and Archives to check the page before saving the settings.
In your case, when you visit a single product, you should see a page where the plugins are disabled according to the last row of this picture https://nimbusweb.me/nimbus-screenshots/7fe700281b881a03445d2e05f65a79cd if you saved the settings that I see on that picture, and of course, if there are no issues with cache and so on.
When you click the preview button, the plugins that are disabled on the row at that moment will be disabled on the preview, but this does not mean on the live site the same plugins are disabled. That is only a preview. If you save the Singles settings, then the disabled plugins will also be those ones that are disabled on the live site, if the SIngles settings are not overridden by other settings.
The first time is complicated. I suggest you read here to understand better the priorities: https://freesoul-deactivate-plugins.com/how-deactivate-plugiins-on-specific-pages/.
What you read on that page is all about saved settings.
Please, let me know if it is still not clear, or if something does not work.
Have a great day!
Jose