This one is quite interesting. Because the `package.toml` file for the `kea` package doesn't specify a build command, it defaults to running `dpkg-buildpackage -uc -us -tc -F` against the cloned git repo. This is a problem because, by default, `dpkg-buildpackage` runs tasks in parallel (the limit is defined as `auto`, but seems to be based off the number of processors the system has), and there's a component being compiled in `kea` that other `make` tasks rely on, but ends up sometimes not being built in time for the dependent task to run successfully.
The easiest fix here appears to be adding a build command to `kea`'s `package.toml` that specifies the additional argument `-j 1` to the original `dpkg-buildpackage` command, which reverts `dpkg-buildpackage`'s behaviour to serial execution of tasks.