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error MSB4057: The target "GetTargetPath" does not exist in the project

Inside Visual Studio 2019, v16.11.7

Getting the following error on a solution that has multiple projects:
Error MSB4057: The target "GetTargetPath" does not exist in the project

A project can be set up to target multiple frameworks, using the following.

<TargetFrameworks>netcoreapp3.1;net48</TargetFrameworks>

This line in the project file would have defaulted to the following:

<TargetFramework>netcoreapp3.1</TargetFramework>

Notice the 'TargetFrameworks' is singular/plural.
With one of the recent VS updates, using the plural variant causes problems. The fix is for ALL the projects in the solution to use the plural variant.