A CopyFilesBuildStep on the macOS build step list copies its file over the built executable instead of next to it, but only when the target is macOS Universal. The single-architecture macOS targets are unaffected.
The build reports success, and codesign -v passes on the result, so nothing signals that the product is unusable.
workaround is to build per platform (arm 64-bit and x86 64-bit)
I entered a bug report at https://tracker.xojo.com/xojoinc/xojo/-/work_items/81927