Here are a couple of other threads that discuss similar issues:
Kamstrup Multical 403 Erratic flow readings - Hardware / Heatpump - OpenEnergyMonitor Community
New emonHP install with strange readings - Hardware / Heatpump - OpenEnergyMonitor Community
In my case it eventually resolved itself, especially after space heating started to run more often as we went into Autumn so it was probably dissolved air that was being released at higher temperatures when doing DHW runs. Hasn’t been a problem since.