Note:

• I don't know whether / what other libraries are affected, and whether Newtonsoft.Jsonis an unusual one.

• The bug also exists in Windows PowerShell.

• The error is ultimately the same as in Working directly with certain .NET types, such as Newtonsoft.Json's, is broken: output formatting #10650: 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.

The problem in a nutshell: when [JValue] instances are implicitly converted to strings, they stringify to the empty string.

By contrast, calling .ToString() explicitly works as expected.

Steps to reproduce

# Parse JSON text into a [JObject] instance.
$obj = [Newtonsoft.Json.JsonConvert]::DeserializeObject('{ "foo": "bar"  }')

# Works as expected:
# Call .ToString() explicitly on the `.foo` property, whose value is a [JValue] instance.
$obj.foo.ToString()

# Problem 1: default output formatting yields '' interactively,
#                   and fails with Out-Default
{ $obj.foo | Out-Default } | Should -Not -Throw

# Problem 2: string interpolation yields ''
"$($obj.foo)" | Should -Not -BeNullOrEmpty

Expected behavior

All tests should succeed.

Actual behavior

Tests 2 and 3 fail.

• Test 2 fails due to an exception, which points to the underlying problem:

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

• Test 3 fails, because (as with interactive output), $foo.bar stringifies to the empty string.

Environment data

PowerShell Core v7.0.0-preview.4 on macOS 10.14.6
Windows PowerShell v5.1.18362.145 on Microsoft Windows 10 Pro (64-bit; Version 1903, OS Build: 18362.295)