Not sure how Unify handles MIDI channels - but you may try placing Montage into single-channel MIDI mode to "feed" Unify something that resembles more what other keyboards tend to do in the MIDI space.

Reading the Unify manual (quickly) - I do not see that you have many MIDI filter options. The MIDI Filter menu lets you filter out sustain CC messages. Which you may try - but that's probably not it because you've probably already tried just not pressing the sustain pedal (or never used it in the first place). But otherwise, filtering out pitchbend or modwheel (the other two filter options in Unify) aren't going to change anything. If this were a standard DAW I'd say to filter out sysex, filter out CCs (all), and also route only one MIDI channel to the VST. The last suggestion (MIDI channel) can be approximated by setting Montage to single-channel MIDI mode.

When I use my MONTAGE as MIDI masterkeyboard with Unify ( a VST- host programm )
There is an issue with DEXED ( a VST DX Plugin ) that in some patches this plugin rings forever ....
The developer of Unify can´t reproduce this
If I use my Genos as a MIDI Masterkeyboard it works fine. Only my MONTAGE has that issue
( I´ll also check it with my MODX7 )

You would use the Zone Master function any time you are connecting via MIDI OUT to other synthesizers or MIDI modules, and you wish to control and manage them from your MONTAGE or MODX. In short, when using the MONTAGE/MODX as your Master Keyboard Controller via MIDI.

You can create a Performance specifically to control your external device or devices. When you say you are using your MONTAGE as a master keyboard can you explain? Are you using the Zone Master function?

If not, that would partially explain your current issue. Let us know.

Mastering MONTAGE: Zone Master FAQ

with MONTAGE in MIDI single mode it is not happening again. I´ll take a closer look

You can either control your VST with a single-PART Performance (and use multi mode) - or you can use a mult-PART performance in multi-mode and turn on Zone Master and also go into each PART and turn the PART level zone control on. In the zone settings, change the MIDI transmit channel to OFF for every zone except the one PART which is meant to control your VST.

In my quick look at Unity - if you have a multi-slave environment (not only your VST as a slave to Montage as a master - but also other slave devices) then Unity lacks the MIDI routing capabilities to make this work in all situations. Using the no-cost licensed version of Cubase AI that was provided with Montage makes for a better VST host in terms of MIDI routing flexibility as you can only pass one MIDI channel worth of data to the VST.

Maybe Unity can do this - but I didn't find the options in a quick scan. And I haven't installed the demo version to try to sort it out.

As the article explains, what is different about the MONTAGE from most keyboards you are probably familiar with is that when just playing it you might be transmitting on more than one channel at a time.

You never answered: Are you using the Zone Master?
We know the answer is no... because when you set the MONTAGE to MIDI I/O = Single you are not. You are settling for whatever the MONTAGE is generating for itself and hoping that it works for your external device

When you want to intelligently control what goes Out via MIDI, you’ll want to learn to setup the Zone Master function — setup a Performance to match what you wish to accomplish. If you want to use both internal and external sounds together, you’ll want to invest time in learning about how to use the MONTAGE as a Master Keyboard Controller.

Extra Credit:
Like every keyboard you’ve ever owned with MIDI, it defaults to sending MIDI data that if captured and played back to itself will reproduce what you played. The Zone function lets you setup and determine exactly what gets sent to an external device. These can be separate. Just FYI.

You never answered: Are you using the Zone Master?

Sorry I am reorganizing my studio / homeoffice and can t reach my MONTAGE at the moment.