Ideally the build scheduler would assign one job to each candidate worker before assigning a second or third job to the same worker. Right now it's possible to have a single a-full with 3 builds running on half the available workers while the other half of the workers sit idle. The problem is more apparent with the increased number of workers in the Valkyrie cluster.
We do customise the build scheduler so this is possibly a bug with how we've done that.
Due to be fixed once we pull in changes from https://github.com/buildbot/buildbot/pull/6286. Blocked on Yocto Project plugin updates.
Wasn't quite that simple but fixed with: https://git.yoctoproject.org/yocto-autobuilder2/commit/?id=754040f52cbf4d65d5c1df22d56f7687486e02a2