A breadcrumb names a section by its title rather than by its directory.
Every page under blog/ sat under a breadcrumb that said blog. The section is called Release Notes, and has been since it existed, in the title of its _index.md — which is what the section heading already showed.
The trail printed p.section, the directory name. That is a URL segment. It is the section's name only when the two happen to be the same word, and the coincidence held long enough to look deliberate: every directory anyone had made was lowercase already, so the trail read as a considered lowercase style rather than as a path printed where a name belonged.
The crumb is the section's title now, lowercased the way a crumb at the content root already lowercased its page title. The link is unchanged — the href is still the directory, because that is what a URL is made of.
<a href="/blog/">release notes</a>Upgrading
A site whose section directories match their titles sees no change. A site where they differ sees the title, which is the point. If a section should be called what its directory is called, that is the title in its _index.md.