Hmm, sorry you're having trouble. Is this with an edited voice, SA voice, or general MIDI voice?? I would check my MIDI settings on both of the registrations. Also double-check the split point, harmony, ensemble, etc. First I've heard of this, the more frequent complaint is an abrupt cut-off or dropped notes when changing voices. Yamaha's arranger development is pretty evolutionary, they don't introduce many new features. Ensemble and PC-based voice editing are two things that I remember being "new" with the T5.
As a worst case, if you can find out what channel the offending note is on, you might be able to set up an external stand-alone foot pedal that sends an "all notes off" message to that channel. A clumsy workaround for a board that originally retailed for $5K, but better than a full reboot in the middle of a performance.