Additional information: I am able to completely break this in the same way in a new project by going into the intermediates folder and deleting the "stringsdata" files. They will reappear after a clean build, but the xcstrings file will show all strings as "Stale" in Xcode, and localized strings added in the code will not appear in the xcstrings file.

SOLVED: The target setting "Use compiler to extract Swift strings" had somehow been set to No.