That’s what I did:
I’m using HALion SE for Trumpet & Piano
I’m using a VST PlugIn called Trilian for Double Bass
I’m using a VST PlugIn called EZDrummer for drums.
To find out a work-around for my slash problem (I’m still convinced that slash-notation is NOT possible if you use a 3rd party VST and not GM or the Yamaha) - so I added another drum set (basic) which I wanted to assign to HALion SE on Midi Channel 3 → screenshot1
I opened HALion and added a GM JazzKit on midi Channel 3 → screenshot2
I opened my HALion Endpoint Setup dialog
…and look what I got on Midi-Channel 3: a number ‘6’ - I can’t change it, I can’t assign it to the drum kit → screenshot3
(For Paul’s info, the project in question is attached in this thread.)
The reason you’re ending up with an entry called “6” is because the expected instrument name data isn’t available for the “Ride Cymbal (Low)” instrument in your drum kit, for some reason. Dorico can’t find the instrument’s data, so it ends up falling back on using the internal ID it has assigned to the instrument instead, which happens to be “6” for this specific instrument.
It appears that the problem is that the routing data in the endpoint setup dialog has got out of sync with the list of instruments in the project. The ‘6’ refers to the internal ID an instrument that no longer exists. I suspect that you had an instrument in the project, connected it to the 3rd channel of HSSE and then deleted the instrument. When you loaded EZdrummer and Trillian, that puts Dorico into ‘manual’ routing mode so it doesn’t make any more automatic changes. I think this means that the reference to the deleted instrument isn’t removed.
However, that shouldn’t prevent you from assigning any other instrument to that channel. I would suggest though that it’s preferable to reset the playback template back to the default and then map the custom instruments again.