Unexpected ConvertTo-Json results? Answer: it has a default -Depth of 2

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Answer

ConvertTo-Json has a -Depth parameter:

Specifies how many levels of contained objects are included in the
JSON representation.
The default value is 2.

Example

To do a full round-trip with a JSON file you need to increase the -Depth for the ConvertTo-Json cmdlet:

$Json | ConvertFrom-Json | ConvertTo-Json -Depth 9

TL;DR

Probably because ConvertTo-Json terminates branches that are deeper than the default -Depth (2) with a (.Net) full type name, programmers assume a bug or a cmdlet limitation and do not read the help or about.
Personally, I think a string with a simple ellipsis (three dots: …) at the end of the cut off branch, would have a clearer meaning (see also: Github issue: 8381)

Why?

This issue often ends up in another discussion as well: Why is the depth limited at all?

Some objects have circular references, meaning that a child object could refer to a parent (or one of its grandparents) causing a infinitive loop if it would be serialized to JSON.

Take for example the following hash table with a parent property that refers to the object itself:

$Test = @{Guid = New-Guid}
$Test.Parent = $Test

If you execute: $Test | ConvertTo-Json it will conveniently stop at a depth level of 2 by default:

{
    "Guid":  "a274d017-5188-4d91-b960-023c06159dcc",
    "Parent":  {
                   "Guid":  "a274d017-5188-4d91-b960-023c06159dcc",
                   "Parent":  {
                                  "Guid":  "a274d017-5188-4d91-b960-023c06159dcc",
                                  "Parent":  "System.Collections.Hashtable"
                              }
               }
}

This is why it is not a good idea to automatically set the -Depth to a large amount.

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