field is not recognized [Update-MgSiteListItem] PowerShell MS Graph API

I want to update a Sharepoint ListItem via the MS Graph API in an PowerShell Script. When I am adding other column´s besides 'Signed' I got the issue that the 'field '...' is not recognized'. Makes no sense, because the other columns are right next to the 'signed' column for which the command works.

for ($i = 0; $i -lt $Item.Fields.AdditionalProperties.length; $i++) {
    if ($Item.Fields.AdditionalProperties.Mitarbeiter_x002d_Name[$i] -eq $UserName) {
        $ItemMitarbeiterName = $Item.Fields.AdditionalProperties.Mitarbeiter_x002d_Name[$i]
        $ItemTeamsValue = $Item.Fields.AdditionalProperties.Teams[$i]
        $ItemId = $Item.Id[$i]
        
        $Fields = @{
            fields = @{
                Signed = $true 
                Teams = $true
            }
        }    
        Update-MgSiteListItem -SiteId $siteId -ListId $NeueMitarbeiterListId -ListItemId $ItemId -BodyParameter $Fields 
    }
}

Thanks for you help

Much Greetings,
Mariano