Hopefully it should fix that issue because the internal data previously wasn’t created, so the routing could not be set (however, the UI appeared to let you set it).
There are two aspects to the fix:
- After changing the routing, any instruments created after that point can now be manually routed
- For old scores where you’ve done manual routing already and the internal data is missing, Play → Load Sounds for Unassigned Instruments will fix it so that you can then route it.