Dan Lee

That doesn't seem right...
I'm unable to repro with my test cases. Can you please file a bug with your scene? Thanks.

Tser
when you try to render to MPlay the shadow matte is not composited in the beauty, I would have thought that you would get the same beauty image which was displayed in the render view and the beauty pass is missing the area where the shadow was

When you have "Add holdout shadows to beauty" enabled offline render is indeed producing the same image you see in the viewport. The reason you see shadows in viewport but not in MPlay is because the viewport has lighter background by default, and MPlay has black background. If you composite the rendered beauty pass in comp (or change the background in MPlay via Load Background... menu and set it to something non-black) you will see the same image.

so the scene cannot react to the shadows in reflections, refraction's etc. I tried to create a shadow catcher node using the Mask by Attribute node which works to a certain extent, but it's not accurate enough to be truly convincing

If you seek accuracy or want to have shadows show up in secondary bounces, I would recommend using background plate workflow instead. The shadow catcher feature was added mostly in response to clients who seek simpler and easier solution where accuracy is secondary concern (e.g. drop shadows for graphic design work). Shadows are an absence of light - it's meaningless to reflect shadow when there's nothing being reflected to take light away from. Basically it's nothing on nothing (or black on black if you'd prefer). Background plate material allows you to project plate to the shadow catching geometry and serve as reflection source whose light can be taken away from (it infers how much shadowing should be present based on the plate colour and radiance, so it produces a lot more accurate shadow colour both in primary and secondary bounces, provided that the lighting set up closely matches the lighting condition in which the plate photo was taken).

This is a side effect of the fact that the shadow catcher itself is a holdout - karma can't distinguish shadow on background/shadow catcher holdouts vs matte holdouts which leads to this problem. Can you please file a bug? In the meanwhile the only workaround I can think of is to render shadow catcher in a separate pass (with shadow casting objects as phantoms).