For anyone that’s experienced this issue, I’ve found a fix. A month ago, I noticed that Synth V Pro would cause the instrumental/waveform and MIDI to desynch with each other. Specifically, it seemed that the instrumental would gradually go faster (as it played) than the tempo that was listed, even though it was the correct tempo. It turned out that placing the MIDI in first before the instrumental would cause it to desynch, which is unfortunate because the MIDI would have the information of the BPM and time signatures (yet only at times for the time signatures).
As I’d found out for myself, the simple and easy fix for it is to place the instrumental/waveform into Synth V Pro and then insert the MIDI after that. The desynching issue has disappeared, thanks to doing that. Although, this now also means that since this is how to fix it, the tempo and time signature changes have to be made manually within Synth V Pro, instead of the MIDI automatically being detected and making those changes. It’s tedious, but it can be dealt with.
WAV format, so that didn’t fix anything. For some reason, placing the MIDI file in first and then the instrumental causes it to desync. This is why I had stated to place the instrumental in first, and then the MIDI, then it won’t desync.
A friend of mine has the same/similar issue, i.e., MIDI and WAV play for different duration. Though, his problem is opposite. His WAV plays much longer than the MIDI imported tracks. That is, his MIDI is 54 measures but his WAV goes on to play to measure 66. Naturally, he did import MIDI before WAV. He generates MIDI and WAV from MuseScore. In MuseScore, they play the same duration. No problem found there. However, the same problem occurs when he imported them to GarageBand. So, I guess there is something there that causes the problem, which is not inherent to Synthesizer V. When I got his MuseScore file and generated MIDI and WAV myself and imported them into Synthesizer V, I didn’t encounter any problem whatsoever. Although he uses Synthesizer V Studio Basic version and I use Pro version, it doesn’t seem that is relevant to the issue at hand (because GarageBand reveals the same problem). Also, he discovered by accident that, when he deleted “Andante” tempo instruction from MuseScore, both of the playing lengths of MIDI and WAV became the same, both in GarageBand and Synthesizer V. Since I can’t reproduce the same problem with my settings, this problem is strange.