It is because you copied the .pdb files as well as the executables. The CLR will look for them when it generates a stack trace to try to give as much info as possible about the stack frames in the trace. The .pdb stores the source file name and line number.
You are supposed to deploy the Release build of your code. That enables optimizations that can make your code run a lot faster. You can still copy the .pdb files for that build, they normally have that debug info stripped. Project + Properties, switch to the Release build, Build, Advanced, “Debug Info” setting. The normal setting here for release builds is “pdb-only” instead of “full”. Which implies that source file and line number is not included. Which makes sense, stack traces tend to be a bit unreliable after the jitter has optimized the code.