I had this after riding up Epic KOM. I did a few reboots and got onto Wahoo who suggested I do a Factory Spin down. Its in a hidden menu on the Wahoo app because it can damage the trainer and should only be done if asked by Wahoo.
It happened after coasting from very high flywheel speeds, and I was able to replicate the issue in Wahoo app, Zwift, and IndieVelo. Neither did it matter if I use virtual shifting or manual shifting,
The issue mostly went away, but has occasionally reappeared after hard climbs. I’ve found that adding a little tension to the belt (usually on or two turns) stops the issue.
There are a few posts on this and I really think Wahoo need to address this. Especially if it is being caused by belt stretch.
I suggest reaching out to Wahoo support.