Hi, I’m using Tripleplay in CB10.5 Pro, I had the same problem initially. The way I fixed it was to disable one of the midi device entries for Fishman Tripleplay in the midi port setup, both input and output.
If you look in the midi port panel, (Studio - Midi Port Setup), you’ll see two entries for Tripleplay in both inputs and outputs. One is labelled Fishman Tripleplay in both input and output, the other is MIDIIN2 (Fishman Tripleplay) and MIDIOUT2 (Fishman Tripleplay). The entries labelled MIDIIN2 & MIDIOUT2 are used by Tripleplay for internal control within the device. Untick both these entries in the visible tickbox in Midi Port Setup and that should do the trick. You may have to exit and relaunch Cubase, I can’t remember. This worked for me in 10.5, hopefully it will be the same in 11.