Document libraries keep reverting to Modern view

Hi all,

For some reason, our document libraries keep reverting to the Modern view. On the Site Collection level, I've already set the experience to Classic by default

Site Collection level

I even run the below powershell script which runs without errors, but the modern experience keeps reoccurring.

Add-Type -Path "C:\Program Files\Common Files\microsoft shared\Web Server Extensions\16\ISAPI\Microsoft.SharePoint.Client.dll"
Add-Type -Path "C:\Program Files\Common Files\microsoft shared\Web Server Extensions\16\ISAPI\Microsoft.SharePoint.Client.Runtime.dll"

$webUrl = 'https://ongcsystems.sharepoint.com/' 
$username = Read-Host -Prompt "Enter or paste the site collection administrator’s full O365 email, for example, name@domain.onmicrosoft.com" 
$password = Read-Host -Prompt "Password for $username" -AsSecureString

[Microsoft.SharePoint.Client.ClientContext]$clientContext = New-Object Microsoft.SharePoint.Client.ClientContext($webUrl)
$clientContext.Credentials = New-Object Microsoft.SharePoint.Client.SharePointOnlineCredentials($username, $password)

# To apply the script to the site collection level, uncomment the next two lines.
#$site = $clientContext.Site; 
#$featureguid = new-object System.Guid "E3540C7D-6BEA-403C-A224-1A12EAFEE4C4"

# To apply the script to the website level, uncomment the next two lines, and comment the preceding two lines.
$site = $clientContext.Web;
$featureguid = new-object System.Guid "52E14B6F-B1BB-4969-B89B-C4FAA56745EF" 

# To disable the option to use the new UI, uncomment the next line.
$site.Features.Add($featureguid, $true, [Microsoft.SharePoint.Client.FeatureDefinitionScope]::None);

# To re-enable the option to use the new UI after having first disabled it, uncomment the next line.
# and comment the preceding line.
#$site.Features.Remove($featureguid, $true);

$clientContext.ExecuteQuery();

Has anyone experienced this as well?