With this workaround I'm not using multi timbral at all. So I don't run into any multi timbral related problems:-).
So how to do it:
1. Dublicate your concert! Cause when Apple fixes the multi timbral issues, you might be able to use the original file again.
2. Make an alias of the multi timbral channel strip. if you have multiple entries in the multi timbral setting, you make multiple aliases. (MainStage: Create an alias of a channel strip).
3. Assign each of the aliases and the original channel strip to the midi channels you need.
As said, depending on the complexity of your setting, this can be very time consuming. But it's at least working for me :-). And by the way: aliases are a good way to make your concert more efficient if you use the same channel strip in different songs.
Hope it helps.