My 2017 2.0TSi hatch is drawing in exhaust fumes at motorway speeds. It doesn't at lower speeds. It's been suggested to me that the exhaust flexi is leaking. However, I'm a bit dubious as the problem only seems to manifest itself after a period on the motorway or NSL road. Any ideas?

Pretty sure its those sorts of speeds that the exhaust temps will be hottest rather than slow/town driving so if there is something that's leaking as a result of heat expansion then that would ring true.

If its a gap in a join it would likely close as everything heats up (I've got a bit of a deep bellow exhaust noise when the car's really cold for the first 10 seconds or so, then it goes never to return) but something in the flex might act differently and open up.