public class ProtocolAddress: ClonableHandleOwner<SignalMutPointerProtocolAddress>, @unchecked Sendable {
  // …
}

This admittedly-lengthy inheritance clause gets formatted as

public class ProtocolAddress: ClonableHandleOwner<SignalMutPointerProtocolAddress>, @unchecked
  Sendable
{
}

which is definitely not the best place for a line break. The general principle here is probably to keep attributes with the thing they're attached to when the attachee isn't at the start of a line.