The reason this is happening is that starting with 0.44.0 Meson got stricter about subproject names. Slashes in the names have never been supported but due to an oversight, it was not a proper error earlier even though it should have been.
- Bug#884627: vibe.d FTBFS with meson 0.44.0-1: Subproject_di... Adrian Bunk
- Bug#884627: Jussi Pakkanen