Update: Based on some feedback from the Apple team, I tried recompiling the .metallib with the Xcode 26 toolchain. It resulted in a new lib that was about 80% larger (7k -> 12k), but worked with both my iOS 26 and iOS 18 devices. I haven't had a chance to test the new lib on older devices yet, but this seems like a solution for people using custom Metal kernels. I'm holding off marking this as answered until I hear back from Apple if they're going to be able to fix compatibility with "old" .metallib kernels.

+1 to this issue. I am working with a couple of different app developers who are getting the same problem.