I have a workaround by using DOpus-Scripting-Extensions (thanks PolarGoose and errante!) until it gets properly fixed in DOpus. I have faith it will eventually.
I created script columns (multicol and infotiponly set to true) for all the necessary values I use in my video tooltip, Replaced all the DOpus supplied metadata keywords with the ones retrieved from MediaInfoLib via DOpus-Scripting-Extensions to the columns. Now I don't get a single blank thumbnail ever. More evidence the problem resides within DOpus.