Issue with Tacx NEO 2T

Hi I have recently bought a Tacx Neo 2T trainer and set it up.

My bike runs an ultegra gear set and I have tried with a 105 cassette and also an ultegra cassette on the trainer. Both the correct speed (11).

My issue is when I come to ride my bike.
When I pedal forwards everything is ok.
However when I begin to coast/free wheel, my chain drops down and is no longer tight. When I have had the trainer on working through the mains power, this obviously causes issues.

Hopefully someone can advise/assist me with the issue.

Thank you