OK, so that didn’t work. I’m really starting to think it’s actually a random bug with the video engine in Dorico rather than anything I’m doing wrong as far as conversion goes, the same video file opens and plays perfectly in Cubase Artist 9.0.30. Ironically the last time I posted about this here the issue was on my laptop, this time it’s on my main PC. 
I don’t know if it’s of any help as far as possible bug testing goes but i decided to go poking around in the files for both Cubase and Dorico and noted that videoengine.dll for each is slightly different:
Whether this has a bearing on my issue I don’t know, I’m not a programmer by trade, but I thought it was interesting nonetheless.
So I guess I’m going to have to do what I did last time; uninstall Dorico, trash all my preferences and start over again. This is only annoying because when everything is working as it should it’s pretty much flawless, synchronisation is rock solid and navigating through the video using the shortcuts is quick and painless… just generally writing to film in Dorico is a joy. Hopefully whatever is going on here can be resolved eventually.