# get the content type from the library
$ct = Get-PnPContentType -Identity "Folder" -List "Documents"

# get the fieldlinks collection from the content type
$fieldLinks = $ct.FieldLinks
(Get-PnPContext).Load($fieldLinks)
Invoke-PnPQuery

# get the title field from the site columns
$titleField = Get-PnPField -Identity "Title"
$fieldLinks.GetById($titleField.Id)

# load the fieldlink for the title field
$fieldLink = $fieldLinks.GetById($titleField.Id)
(Get-PnPContext).Load($fieldLink)
Invoke-PnPQuery

# make the title field visible
$fieldLink.Hidden = $false
$ct.Update($false)
Invoke-PnPQuery