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

Inizia riproducendo il malfunzionamento del completamento in VS Code con la funzione PowerShell fornita, quindi esamina i log allegati usando docs/troubleshooting.md. Confronta il completamento al secondo inserimento per ValidateSet, Register-ArgumentCompleter e i parametri enum. Il lavoro è completato quando queste origini del completamento funzionano in modo coerente tra inserimenti successivi e un test di regressione copre il malfunzionamento.