I don't know if it's already fixed (I've seen some similar posts but they weren't very helpful), but I have so many problems with rendering, the video often freezes randomly in the rendered video, and the audio keeps normal, when this happens it stops the sync and everything becomes crazy, I can't use the app with that
I was testing the free version by the way
I use a Late 2014 Mac Mini, HD, 8GB DDR3 Dual-Core Intel i5 in case it is needed
I was not using any footage, only the midi itself.
I had the exact same problem, when I rendered video with 4k resolution, the video was freezing randomly (the sound was normal). Turned out that the problem was my video card. I updated my video card's drivers and everything works now.
Hope this helps.