Inconsistent behaviour of lang.Merge (regression introduced in v0.62.0)

This is yesterday’s question stripped down to a bugreport; I thought I’d run it on the support forum before rushing to open an issue on GitHub.

See this git repository for a minimal site examlpe.

On the example site, layouts/index.html and layouts/rss.xml make use of lang.Merge. Despite the related lines being exactly the same, when actually built with hugo the newest post in index.xml lacks .Content, whereas en/index.xml, index.html, and en/index.html all have .Content displayed for all three posts.

When in development mode (hugo server) the issue is only present occasionally.

OK, further investigation showed that this is actualy a regression introduced in 0.62.0 by this commit.

Reported as