Powershell issue with executing in Task Scheduler

I have below script that add users to o365 group at start of their work as below:

$DateMaxTime = (Get-date).AddDays(0)
$DateMaxTimeNew = (Get-date).AddDays(-30)

$usersRO = Get-ADUser -Filter * -Properties * -SearchBase "OU=Users,OU=Resources,OU=Romania,OU=DataManagement,DC=USA"|where {$_.Description -like "*TEMP*" -or $_.Description -like "*PERM*" } |select samaccountname,description,name
$groupsRO = '#O365-EXTERNALACCESS'


$FinalResultRO = New-object System.Collections.ArrayList

ForEach($groupRO in $groupsRO){
$membersRO = Get-ADGroupMember -Identity $groupRO -Recursive | Select -ExpandProperty samaccountname
Foreach ($userRO in $usersRO){
$AcountNameRO = $userRO.samaccountname
$DatePartRONew = get-aduser -identity $AcountNameRO -Properties * | Select-Object whenCreated
$DatePartSubsRONew = $DatePartRONew.whenCreated
$DataPartROdesc=$userRO.description
$expressionRO = ([regex]'(\d{2}/\d{2}/\d{4})').Match($DataPartROdesc).Groups[0].Value
$DatePartRO= $expressionRO
$FinalDateRO = [datetime]::ParseExact($DatePartRO,'dd/MM/yyyy',$null)
    If ($DatePartSubsRONew -lt $DateMaxTimeNew){
    Write-Host "$AcountNameRO ouf of date scope"}
    else {Write-Host "$AcountNameRO in scope"
        If ((get-date $FinalDateRO.Date) -eq (get-date $DateMaxTime.Date)){
            Write-Host "$AcountNameRO is a today Starter"
            If ($membersRO -notcontains $AcountNameRO ) {
            Write-Host "Adding external group $groupRO for: $AcountNameRO"
            Add-ADGroupMember -Identity "#O365-EXTERNALACCESS" -Members $AcountNameRO 
            $FinalResultRO.Add((New-Object psobject -Property @{User=$AcountNameRO}))
             } 
            Else {Write-Host "$AcountNameRO  exists in  group $groupRO"}
        }Else {Write-Host "$AcountNameRO is not a Starter"}
        }
    }
}

$listRO = [array]$FinalResultRO |Select User |Out-String
$listRO.gettype()

if [string]::IsNullOrEmpty($listRO){
Write-Host "nothing to send"
}
Else {
Write-Host "Mail sent"
Send-MailMessage -From "[email protected]" -To "[email protected]" -Subject "Following users have been granted external access rights" -smtpServer "donut" -body "$($listRO)"
}

I run this script daily in task scheduler with higest privilage .

For some reasons, sometimes when script is executing , telling me that users has been added to group but its not changing in Active DIrectory . Only when I run the script second time its working (manually on powershell , not using task scheduler).

What can be a reason for this ?