SDK-style SQL database project does not include assembly in dacpac although building with .NET Framework

- SqlPackage or DacFx Version: Microsoft.Build.Sql 2.1.0
- .NET Framework (Windows-only) or .NET Core: both
- Environment (local platform and source/target platforms): local windows

[The documentation states](https://learn.microsoft.com/en-us/sql/tools/sql-database-projects/sql-database-projects#original-projects-vs-sdk-style-projects) that SQLCLR objects are supported when building the project with .NET Framework. However this doesn't seem to work.

**Steps to Reproduce:**

1. Create an SSDT project in Visual Studio.
2. Add an SQLCLR object

3. Build and find the assembly included in the dacpac
4. [Convert to SDK-style project following the manual](https://learn.microsoft.com/en-us/sql/tools/sql-database-projects/howto/convert-original-sql-project)
5. Build with .NET Framework (MSBuild.exe) and find the assembly not included in the dacpac anymore.

**Did this occur in prior versions? If not - which version(s) did it work in?** Don't know.