reflex, here are my steps to reproduce this behavior in FL 20. I did these steps in an empty project (no plugins/MIDI/effects/etc.)
1. Pull up Sytrus in channel rack (I prefer to use Plugin Picker to do this)
2. Playlist -> Track mode -> Instrument track -> Use existing channel -> Sytrus
Sytrus will be set to Channel 1.
3. Option A: [Clone the original Sytrus] or Option B: [add a new Sytrus instance to channel rack + set its mixer track to 1]
4. Playlist -> Track mode -> Instrument track -> Use existing channel -> Sytrus #2
Sytrus #2 and Sytrus are now both set to Channel 1 of mixer.
I don't think this behavior is specific to Sytrus, but Sytrus is my main instrument plugin, so I used it for the demo.
I did this experiment after downloading/installing FL Windows FL Studio 20.1.2 (February 2019). However, "About" says I am running FL v20.8.3. I am not sure what is going on here.
When I repeat the above steps of my experiment using the beta, whether I use option A or option B, Sytrus is set to channel 1 and Sytrus #2 is set to channel 2.
I can do a 5th step in the beta: Sytrus #2 -> Unassign from instrument track -> set to mixer slot 1 -> repeat step 4. Again Sytrus #2 will be moved from mixer slot 1 to mixer slot 2.