ValidateSet/Register-ArgumentCompleter/Enum completions does not work correctly

Issue Description

I am experiencing a problem with... completions failing for 2nd entry.

Notice that it works on 14th line, and then very same command fails on line 10th.

function Get-GPOZaurrPermissionSummary {
    [cmdletBinding()]
    param(
        [validateSet('AuthenticatedUsers', 'DomainComputers', 'Unknown', 'WellKnownAdministrative', 'NotWellKnown', 'NotWellKnownAdministrative', 'NotAdministrative', 'Administrative', 'All')][string[]] $Type = 'All',
        [validateSet('Allow', 'Deny', 'All')][string] $PermitType = 'All',
        [ValidateSet('GpoApply', 'GpoEdit', 'GpoCustom', 'GpoEditDeleteModifySecurity', 'GpoRead', 'GpoOwner', 'GpoCustomCreate', 'GpoCustomOwner')][string[]] $IncludePermissionType,
        [ValidateSet('GpoApply', 'GpoEdit', 'GpoCustom', 'GpoEditDeleteModifySecurity', 'GpoRead', 'GpoOwner', 'GpoCustomCreate', 'GpoCustomOwner')][string[]] $ExcludePermissionType,
        [Microsoft.GroupPolicy.GPPermissionType[]] $Test,

        [alias('ForestName')][string] $Forest,
        [string[]] $ExcludeDomains,
        [alias('Domain', 'Domains')][string[]] $IncludeDomains,
        [System.Collections.IDictionary] $ExtendedForestInformation,

        [string] $Separator
    )

I tried to replicate it to give you reproducible code but I just can't. I'm experiencing it across all my modules and I can't pinpoint it to what I'm doing wrong. It fails the same way for both [Microsoft.GroupPolicy.GPPermissionType[]] which is an enum, but also have the same issue with ValidateSet and Register-ArgumentCompleter that makes me want to cry :-)

Attached Logs

1601583262-5511c227-6bca-4824-9bad-6f29a894ed501601499246405.zip

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Environment Information

Visual Studio Code

PowerShell Information

Visual Studio Code Extensions

Start by reproducing the completion failure in VS Code with the provided PowerShell function, then review the attached logs using docs/troubleshooting.md. Compare second-entry completion for ValidateSet, Register-ArgumentCompleter, and enum parameters. Done means these completion sources work consistently across successive entries, with a regression test covering the failure.