Exception calling super in Objective-C class on Big Sur

Thanks for the very informative session (Advancements in the Objective-C runtime).

I'm seeing an exception in a shipping app (FB7781503) on the WWDC20 Big Sur build when calling super from an NSLayoutManager subclass.

Code Block objective-c
- (NSArray<NSRulerMarker *> *)rulerMarkersForTextView:(NSTextView *)aTextView paragraphStyle:(NSParagraphStyle *)paraStyle ruler:(NSRulerView *)aRulerView {
    NSArray <NSRulerMarker *> *markers = [super rulerMarkersForTextView:aTextView paragraphStyle:paraStyle ruler:aRulerView];
		...
}

The layout manager instance data doesn't seem to have been corrupted prior to calling super, but calling super throws

Code Block 
-[STLayoutManager rulerMarkersForTextView:paragraphStyle:ruler:]: unrecognized selector sent to instance 0x7fce12e697b0 

I'm wondering if the runtime changes are the cause. I'm able to reproduce this. Any tips for debugging this one?

Steve