Powershell : Tabular Object Model, model object is null

I sometimes use PowerShell to quickly alter a set of similar measures in a Power BI dataset. This could be done with this code :

[System.Reflection.Assembly]::LoadWithPartialName("Microsoft.AnalysisServices.Tabular")

$serverName = "localhost:xxxxx";
$databaseName = "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx";
$svr = New-Object Microsoft.AnalysisServices.Tabular.Server
$svr.Connect($serverName)
$db = $svr.databases.GetByName($databaseName)

$findstring="find string"
$replacestring="replace string"

foreach($table in $db.Model.Tables) {
  foreach($measureobj in $table.measures) {
  $measureobj.expression=$measureobj.Expression.replace($findstring,$replacestring)
}
}

#$db.Model.SaveChanges()

Works great, until recently I found some reports where the Model-attribute of the database seems to be empty.

 Although it works perfectly fine in PowerBI, I can't find the model anymore in PowerShell. Also Tabular and DAX Studio are working fine with the reports dataset.