Note:

• I don't know whether / what other libraries are affected, or whether Newtonsoft.Json is an unusual one.

• The bug surfaces only if the input JSON text contains an array.

• The bug is a regression from Windows PowerShell, where it doesn't surface.

• The error is ultimately the same as in Working directly with certain .NET types, such as Newtonsoft.Json's, is broken: implicit stringification #10652: Target type System.Collections.IEnumerator is not a value type or a non-abstract class. (Parameter 'targetType') - note how the type name is an interface, not a class.

Steps to reproduce

# Parse JSON text into a [JObject] instance.
# Note: To surface the bug, the text must contain an ARRAY
$obj = [Newtonsoft.Json.JsonConvert]::DeserializeObject('{ "foo": [ "bar" ] }')

# Apply default output formatting, which fails due to the bug:
{ $obj | Out-String } | Should -Not -Throw

Expected behavior

The test should succeed.

Actual behavior

The test fails, because the implicit output formatting threw an exception.

Target type System.Collections.IEnumerator is not a value type or a non-abstract class. 
(Parameter 'targetType')

PowerShell Core 7.0.0-preview.4