Reproducible by the same steps mentioned in the description for Chatto project.
P.S. Chatto is part of Swift Source Compatibility Suite and, usually, it helps us to catch problems in advance. Maybe this is a good idea to have a set of checks but for the module stability feature as well.
I have a barebones sample project that reproduces the issue. It includes pre-built binaries for easier testing, and is simpler relative to Chatto that dive (JIRA User) linked above, and self-contained unlike the sample project from the original bug posting.
Does anyone on the Swift team have insight into whether this issue is on anybody's radar for investigation?