This was due to my usage of UIActivityViewController in my SwiftUI views.
When relying on UINavigationController in a SwiftUI view, you can use the .toolbar modifier to add/manage navigation bar button items. I did this for my 'share' buttons, which would allow the user to share a link to my content via other apps on their phone. My solution mirrored the solution in this github repository, which is as follows:
Place a UIViewRepresentable in the background of your SwiftUI share button
Representing a custom, empty UIViewController
The custom UIViewController presents a UIActivityViewController when its var item: Binding<ActivityItem?> is given a value
give it a value when your button is tapped
Something about that view configuration totally broke the SwiftUI <-> UIKit bridging in iOS 18.4. Still not sure whether that was intentional, and since there is no deliberate messaging in the changelog I assume it was not intentional. If anyone else is having these problems, I hope this helps.
FWIW, I was planning to update my 'share' features to use ShareLink anyway, as I just recently increased the min deployment target to iOS 16, so this was a good reason to prioritize that work. That is what I would recommend to anyone else having this issue.