packages folder contents being marked for check-in

I've got a Windows Forms (.NET 4.8) application which is under source control on dev.azure.com. The project contains a packages folder set up by nuget but installed packages are being marked as pending check-ins despite having a .tfignore file in project root expressly excluding them. I can't understand why.

Here is Source Code Explorer:

I have the following .tfignore file in the Project root (Ice folder):

\packages
!\packages\repositories.config
\.vs

I have "Allow NuGet to download missing packages" and "Automatically check for missing packages during build in Visual Studio" both checked under Options.

I'm using Visual Studio 2019 Professional.

The project file is the legacy (ToolsVersion="12.0") format as I don't believe VS supports Windows Forms apps with the new version.

Any ideas why I can't get the folder to be ignored? Thanks guys.