Not on my main comp so I can't check, but I'd guess this is happening because Valve probably changed the scattergun and rocket launcher from v_models (viewmodel only files) and w_models (world model only files) to c_models (the same model file is used for viewmodels and worldmodels) in the parachute update, so the model files the demo is looking for aren't there.
You can check this by just looking through the TF2 vpks to try to find the rocket launcher models, if it's something like c_rocketlauncher then read on. If it's not, then idk what's wrong.
If you can find backed up models for the rocket launcher, you could probably just plop it in and the demo should find it. Old skins that remodeled the world model of the rocket launcher would probably also work.
If you can't find any of those, you could try hexing the current c_model (see: http://source.gamebanana.com/tuts/8003 ) that's in the current tf2 vpks to make the model files match what the demo is looking for. Can't guarantee this will work.
This is from my limited knowledge of how modeling for source works, so I might just be leading you to nothing. I have no clue if this turns out to not be what's wrong.