stdlib : delete_undef_values doesn't seem to work under Puppet 6

Basic Info
Module Version:  4.x and 5 
Puppet Version: 6.0.0
OS Name/Version: Any

The following code:

$array = [1,2,3,undef,4]
$cleaned = delete_undef_values($array)
notice $cleaned

Desired Behavior:

Should produce:

Notice: Scope(Class[main]): [1, 2, 3,4]
Notice: Compiled catalog for macbook-pro.local in environment production in 0.03 seconds
Notice: Applied catalog in 0.02 seconds

Actual Behavior:

But it produces:

Notice: Scope(Class[main]): [1, 2, 3, , 4]
Notice: Compiled catalog for macbook-pro.local in environment production in 0.03 seconds
Notice: Applied catalog in 0.02 seconds

Please take a moment and attach any relevant log output and/or manifests. This will help us immensely when troubleshooting the issue.