default font, which you can set by preferences > editing > (clicking font-like-ish icon which i think super confusing and annoying), is not working soon as you use scrivening mode.
scrivener has quite ok features with promising directions however unbelievably lots of bugs make this software very difficult to use. if anyone considers to buy scrivener, you would better to try another alternative, seriously
The default formatting system does work with Scrivenings mode and has done for several years, so if you’re finding a problem with it, then it’s most likely either that you have a very unusual combinations of settings (e.g. something about the font you’re using, or the language etc) and have found an obscure bug, or that you are missing out a step in the process.
The key point is that changing the default font works for new documents only, so you have to convert previous documents to the new formatting. This doesn’t happen automatically — you have to choose to do it.
So, to make sure that all the documents in the Binder are using the same default font (and will therefore appear so in Scrivenings):
Change the default font: either in Preferences > Editing > Formatting (for new documents in this and any new projects), or in Project > Project Settings > Formatting for new documents in this project alone. Make sure the Use different default formatting for new documents in this project is ticked).
Then select all the documents in the Binder which you want to be updated to the new default formatting and select Documents > Convert > Text to Default Formatting
Now, when you select the documents for a Scrivening, they should all be in the same new default format (except for paragraphs which already have a different named style, of course).
If you follow that procedure and you’re still getting the same problem, then please get in touch directly with support and given them more details of your setup and an example project.
I’d like to add to Brookter’s comprehensive explanation that scrivening titles have their own font settings (Preferences/Appearance/Scrivenings/Fonts). Which of course isn’t a bug either.