Runn speed locks in Zwift

The speed reported by my Runn unit in Zwift keeps locking partway through a run. If I'm running at 12km/h and change my pace to 13km/h or 11km/h, Zwift still says I'm running at 12km/h.

I can't figure out what causes it, but it happens in 90 per cent of my runs.

Any help would be appreciated.