SwiftPM Objective-C++ builds successfully but Swift shims missing symbols

I have a large Objective-C++ module with declarations such as this:

// ExampleView.h
@interface ExampleView : UIView <UIGestureRecognizerDelegate> { /* literally empty */ }

// ExampleView.mm
#import "ExampleView.h"
@implementation OnboardingAppView

The only additional shenanigans used are assets (defined in the target's resources) and importing a Swift module with C++ interop enabled (via @import only in source files).

However when importing that Objective-C++ module into a Swift module, most of the declarations (including very simple ones such as the one above) are missing.

They are recognized as part of the module (trying to use them correctly adds a fix-it to import the Objective-C++ module) but once imported they're still reported as missing and prevent compilation.

Since this is a massive target, I still haven't figured out what the underlying issue might be. I was hoping maybe somebody encountered this issue and might know what is causing the symbols to go missing?