Dynamic Parameter Tab Completion Bug?

Test-DynamicParameter.ps1

function Test-DynamicParameter
{
    [CmdletBinding()]
    PARAM
    (
        [string[]]
        $Param1,

        # this parameter seems to cause the problem - if I specify [string] then tab-completion works for -DynamicParameter
        [PSCredential]
        $Param2
    )

    DYNAMICPARAM 
    {
        if ($Param1 -contains "a")
        {
            $Name = "DynamicParameter"
            $ParamAttr = [System.Management.Automation.ParameterAttribute]::new()
            $ParamAttr.Mandatory = $true
            $Attributes = [Collections.ObjectModel.Collection[System.Attribute]]::new()
            $Attributes.Add($ParamAttr)
            $Parameter = [System.Management.Automation.RuntimeDefinedParameter]::new($Name, [string], $Attributes)
            $Dictionary = [System.Management.Automation.RuntimeDefinedParameterDictionary]::new()
            $Dictionary.Add($Name, $Parameter)
            $Dictionary
        }
    }

    PROCESS
    {

    }
}

testing.ps1

# this works as expected - hitting tab expands the parameter "-DynamicParameter"
Test-DynamicParameter -Param1 'a' -<tab>

# this fails - hitting tab does nothing. however, the parameter still works if I type it out
Test-DynamicParameter -Param1 'a' -Param2 $Credential -<tab>

# If I change the type attribute of Param2 to [string] then tab-completion works for the dynamic parameter
# Weird!