I imported a MIDI track, rearranged it, changed the volume of VSTi Instrument but that volume keeps changing by itself when it plays a new part (DAW: Cubase).
What is going on?
Your MIDI parts contain CC (Controller Change) and PC (Program Change) messages at the start of each clip.
You need to ignore/remove the CC/PC information.
In Cubase you can do that during recording with the Input Transformer, and you can remove it from clips afterwards in the Logical Editor, or the Key Editor.
Here is how to do it using the Key Editor (notice the CC #7 Volume - this is most likely the culpritt). Simply delete the information or set the value as desired.: