Happened to notice that the logs are full of entries like

client topology not compatible with test (requires at least libmongocrypt version 1.19.0 but using version 0.0.0)

This turns out to be because we're fetching libmongocrypt at a specific commit to work around a cmake bug; the libmongocrypt build script checks for tags or release branches to compute the version number that's baked into the source, and for the single-commit checkout neither of those exists, so it defaults to "0.0.0".