Tentatively, updating to 15.4.1 seems to have fixed this issue with Xcode 16.0 as well.

This is also the case for me. Everything ran great with Xcode 16.0 on Sequoia 15.2, breakpoints in the simulator and all. I updated one of my machines to 15.4 and breakpoints no longer work, getting the message Couldn't find the Objective-C runtime library in loaded images and the debugger crashes. Not able to update to 16.3 at the moment.