See repro below.

As an aside (that will probably become a docs issue): What is the purpose of ConvertTo-Xml? The docs suggest that it is the in-memory counterpart to Export-Clixml, however:

• It produces different XML representations,
• which, if saved to a file, cannot be re-imported with Import-Clixml

Steps to reproduce

[pscustomobject] @{ a = [pscustomobject] @{ aa=11; bb=22 } } | ConvertTo-Xml

Expected behavior

Should output an [xml] instance that is the serialization of the custom object.

Actual behavior

The following error occurs:

ConvertTo-Xml : Unexpected end of file has occurred. The following elements are not closed: Objects.

• If you use -As String, you don't get an error, but the XML string output too is missing the closing </Objects> tag.

• Removing the 2nd property (bb) from the nested custom object makes the problem go away.

• Using [hashtable] instances instead of [pscustomobject]s makes the problem go away.

• Export-Clixml does not exhibit the problem.

Environment data

PowerShell Core 6.2.0-preview.1 on macOS 10.14.1
PowerShell Core 6.2.0-preview.1 on Ubuntu 16.04.5 LTS
PowerShell Core 6.2.0-preview.1 on Microsoft Windows 10 Pro (64-bit; Version 1803, OS Build: 17134.345)
Windows PowerShell v5.1.17134.228 on Microsoft Windows 10 Pro (64-bit; Version 1803, OS Build: 17134.345)