I had the same problem once (Kickr 2018) several days ago … speed never went back to 0 and then spindown failed by Timeout … I used Wahoo Fitness, performed Spindown there, no problem - it finished OK. Then I did spindown in Zwift and it worked (and still works) fine …
you can see my original post here … Kickr spindown - Zwift .vs. WahooFitness - any difference?
GdF