I'm working on a document using Source Serif 4 Regular for body text, and Source Sans Pro for captions and headings. Today I decided to try out Source Serif 4 Display for top-level headings, and activated the 12 fonts. I ended up using Semibold Italic for the headings, and created a paragraph style for it.
From then on, every time I applied the body text paragraph style, the text would be in Semibold Italic instead of Regular, and any changes I did in the Paragraph style options for the two styles using Source Serif 4 would not preview, and would not affect the text after clicking OK. I was able to change the font manually using the Control panel.
Styles using Source Sans Pro were unaffected, and worked properly.
I did the usual troubleshooting – exporting to IDML, resetting preferences (deleted InDesign Defaults and Saved Data), restarted my computer, but to no avail.
I began to suspect that something was wrong with the Source Serif 4 Display fonts, so I deactivated all of them except for Semibold Italic, and that did the trick.
I'm still not sure if the fonts are causing the problem, or if it's an InDesign bug.