I am seeing the exact same thing on an Ultimaker 3. It was printing beautifully and then suddenly started jamming. I did all the same thing you listed, new tubes, new print core, cleaned the feeder multiple times only to have it repeatedly fail. Eventually I noticed the same thickness of the filament as your photo. I just logged in to see if there was a suggested solution for this. I’m going to replace the front fan and will update after testing.