I have seen this with the KicKR V6. I’ve found its not isolated to virtual shifting but also happens with manual shifting.
It happens when coasting from very fast flywheel speeds (power doesn’t matter) and it happens in Zwift, Wahoo App, and in indieVelo.
Wahoo advised, I do a factory spin down and the issue mostly went away. I also found that nipping the belt tension up stops the problem.
There is another post on it here and also on Reddit.
I suggest reaching out to Wahoo and reporting back. Especially if it is caused by poorly tensioned belts