VS 2019: running an extensibility project (VSX) always compiles an unknown external project

Question

Basically this:

1. I created a fresh project in Visual Studio 2019 using the VSX c# template.
2. I moved a bunch of files from an older VSX project (for VS 17).
3. I compiled and ran the project - errors started emerging.
4. I addressed one error, ran the project - got another error.
5. I started addressing the second error and in the process of fixing it I noticed that now VS is for some reason compiling an unknown external project whenever I run my project. I realized it because once the program hit the error and instead of jumping into my code and showing me the line, it showed me this message: "The source file is different from when the module was built. Would you like the debugger to use it anyway?" Also whenever I changed something in my code files now - the changes never apply. Moreover, the first error (which I have already fixed) resurfaced again, and when the debugger tried showing me the place of the error - it clearly used the new file, thinking that it's an old one (even if I change the new file to have empty space in the line where the error was, the debugger will still stand on that empty line, talking about the error).
6. Now whenever I create a new VSX project, even an empty one, and compile/run it - the studio runs that unidentified external project (I have absolutely no clue where that project is, even the files that the debugger uses have no influence on that project.

UPDATE: I deleted Visual Studio cache folders (C:\Users\xxx\AppData\Local\Microsoft\VisualStudio\16.0_xxxx) as well as reset the experimental Visual studio with the start menu prompt "Reset Visual Studio 2019 Experimental Instance". Now, when I compile my VSX projects, a clean and empty Visual Studio instance is launched - my extension code is ignored.