Here's a successful workaround for something very much similar to what you describe. When my 'Solution' contained only .NET Standard projects and I attempted to Debug using an external 4.6.1 Framework executable (let's call it "Foo.exe" from a separate solution) my breakpoints weren't hitting either.
My understanding is that there are two distinct debuggers, the .NET Core Debugger and the Full Framework Debugger. My vs 'Solution' defaulted to the former since that's the only type of Project that was in it.
What eventually worked for me was tricking VS2019 into using the Full Framework Debugger.
To do this, I added a placeholder 4.6.1 Framework console project to the solution and set it as the Startup project. In the Debug tab I set the Start Action exactly the same, pointing the value in Start External Program to the same one as before ("Foo.exe"). Now the breakpoints work.
NOTES for clarity:
- The placeholder app doesn't do ANYTHING and is never ever actually run.
- It doesn't need to reference the other .NET Standard assemblies that you're trying to debug.
- The "Foo.exe" is the debugging process that gets attached in any case, but the breakpoints are only going to get hit when the Startup Project is a Framework project and NOT when the Startup Project is set to one of the .NET Standard projects in the same solution.
BTW setting the "Build\Advanced\Debugging information" to 'Full' as described in the earlier posts made no difference in my scenario but THANK YOU it made sense and was sure worth a try!