Note: I can only reproduce this reliably on macOS 10.15.4.
I see it occasionally on Ubuntu 18.04, and can never reproduce it on Windows.

It may be connected to using Start-Sleep inside the script block; not sure if the fact that a custom class is used across thread boundaries is relevant [update: it is]

Steps to reproduce

class Foo { static [object] Echo($val) { return $val } }
$cls = [Foo]
while ($true) { 
  $res = 1..10 | ForEach-Object -Parallel {
    Start-Sleep -ms 100; ($using:cls)::echo($_) 
  }  |  Sort-Object
  Compare-Object $res (1..10) | Should -BeNullOrEmpty
  Write-Host -NoNewline . 
}

Expected behavior

The loop should run indefinitely, because the test should alway succeed.

Actual behavior

The loop eventually - after a varying number of iterations - breaks due to intermittent test failures, stemming from a missing output; e.g.:

Expected $null or empty, but got @(@{InputObject=5; SideIndicator==>})

That is, not all input objects were echoed.

Environment data

PowerShell Core 7.1.0-preview.3  on macOS 10.15.4