I'm using the latest version of PrimeNG(6.0.1) and Avalon (6.0.0) with the new primeng icons (1.0.0-beta.9). Since the new version of primeng doesn't require font-awesome 4, I removed it (looking to upgrade to font-awesome 5). However, the accordion control's expand/collapse icons don't display without font-awesome 4 installed. Note that all other controls (at least the one's I'm using) display the new primeicons correctly.
Also, attempting to change the icons via the expandIcon and collapseIcon attributes had no effect.