How to run multiple parallel SandboxProcesses

I'm trying to run multiple MSBuild processes in parallel via SandboxedProcessFactory. Each process is given a different FileAccessManifest to block access to certain directories (which vary from process to process). I'm finding that the blocking is inconsistent such that it looks like the processes are stepping on each other. I get random blocks pretty reliably on some portion of the processes (always changing), but if I retry with the same manifests, they succeed. One other observation is that if I run the processes serially, everything works without issue.

My sandboxed process info looks like this:

var info = new SandboxedProcessInfo(
    fileAccessManifest.PathTable,
    null,
    msbuildPath,
    disableConHostSharing: true
    loggingContext: new LoggingContext(nameof(BuildXLMSBuildInvoker)))
{
    Arguments = arguments,
    PipDescription = "Run MSBuild",
};  

Is there a way to isolate the sandboxed processes from each other?

Start at SandboxedProcessFactory and SandboxedProcessInfo, using the reported setup to reproduce parallel MSBuild runs with distinct FileAccessManifest instances. Compare the parallel and serial cases; done means the isolation behavior is understood and the issue has a documented resolution or a confirmed reproducible defect.