I don't have any experience of this directly, but this is exactly the question/problem that comes up here repeatedly.
There are a number of the midi/usb adaptors on the market, and some are a lot better than others. Not sure if it's the design of the firmware in the adaptor, or the speed of the chips (it needs only 1 to be VERY slow) , but basically the midi data is not getting through 100%, bits are being 'lost', so notes are being lost, or NOTE OFF commands are not getting through. In your case, it's the OFFs, maybe they are more prone to hit the interface in a lump?
Better interfaces have been recommended in the past, and these seem to suddenly solve the problem.
The information you give suggests that the interface works fine up to a point, so seems everything else is OK. Past that certain point, the unit does not cope. Just needs to be faster?